Topic: 71 cb750 engine problems

[orkid1989]

im having some problems with my cb750. first it has almost no power. it has a hard time getting over 60 on the highway plus it keeps over heating. any ideas?

[nancy]

Possible poor fuel delivery...pull and check state of each spark plug - sooty? White?

[orkid1989]

ill have to check after the engine cools down. the carbs are freshly cleaned so i doubt that. could it possibly be the jets? right now it has stock jets with pod filters and a 4 to1 exhaust. also i checked the compression yesterday and it was 100psi so maybe thats where the problem is?

[azporkey]

These bikes are to expensive to fool around with .You may ruin the engine. take it to a good Honda shop Before something bad happens to it. LOL az

[MoMo]

how did you check compression, hot engine and throttle wide open or cold? and were the carbs completely disassembled when cleaned?  If the plugs are white, stop riding before you do serious damage .good luck , Larry

[orkid1989]

These bikes are to expensive to fool around with .You may ruin the engine. take it to a good Honda shop Before something bad happens to it. LOL az

well i tried going to the honda dealer and they wont work on my bike because it is too old and their isnt any good shops around here that will work on it or i would have already had it done.

how did you check compression, hot engine and throttle wide open or cold? and were the carbs completely disassembled when cleaned?  If the plugs are white, stop riding before you do serious damage .good luck , Larry

i checked the compression by taking out the spark plugs and connecting the pressure gauge and pumping the kick start a few times till the pressure stops rising. the engine was slightly warm when i did it. is that the right way to check it?  the carbs were left on the bracket holding them together but other than that i took them completely apart then installed them and synced them up

[andy750]

These bikes are to expensive to fool around with .You may ruin the engine. take it to a good Honda shop Before something bad happens to it. LOL az

Hopefully this guys just joking as what he is saying is BS. Much cheaper to do the work yourself than take it to a dealer. Anyway...what colour are your plugs - are they all the same colour? What size jets (pilot/main)? Have you set the timing? Valves? Sync the carbs after cleaning them?

Your engine is likely fine and more likely its fuel related/ignition related.  Got a manual? If so check basic tuneup section. Ask questions - photos help to diagnose as well.

Do yourself a favour and google "compression check motorcycle cold"


Good luck
Andy

[MoMo]

throttle should be open when kicking, and with a warm engine compression should be 150. 120 psi or so with cold engine. But I doubt that is your problem, if compression is low engine would not start.  Have you set timing with a light, spark advance may not be working.  Larry

[Spanner 1]

As our esteemed friend TT would say; " When was the last 3,000 mile tune-up done ? ".

[Hondawggie]

ill have to check after the engine cools down. the carbs are freshly cleaned so i doubt that. could it possibly be the jets? right now it has stock jets with pod filters and a 4 to1 exhaust. also i checked the compression yesterday and it was 100psi so maybe thats where the problem is?

If you are 100% sure you have stock jetting and you have pods, the pods modification now allows the engine to pull in a lot more air but with stock jets the fuel flow is little changed to match that increase in air flow.  I think your plugs will not look light tan when you check them.  With the addition of a 4-into-1 you need bigger mains, probably bigger pilot jets too.  And maybe move the clip on the jet needle downward 1 notch.

(SYMPTOM: Overheating and low power)  +  (POSSIBLE CAUSE: stock jetting + pods + 4-into-1) = low power and overheating.

As someone said it sounds like a tough case and dangerous to ride the bike if it is that bad.

If you find time you can
- check points gap and timing
- check valve clearances
- check and probably change jetting

When you cleaned the carbs did you check/set the float level?

If you're getting 100psi *and* you had the throttle all the way open while you were kicking it, man that is kinda low compression, would think you'd burn oil.  I suspect when you did your compression test the throttle was closed...?

EDIT:  the shop manual showing how to check the stuff above is under 'Download Manuals' at the top of this forum or click here => http://forums.sohc4.net/index.php?topic=17788.0


Then scroll down a little and click on this to download the manual:

"CB750K CB750F Complete Service Manuals, Parts List Manuals & more 1969-1978 (HUGE FILE): http://www.mediafire.com/file/nzkmzodmgin/Honda CB750 1969 - 1978 Parts Lists - Service Manuals & More.pdf"

[Magpie]

i had a similar problem years ago and it was to do with how I routed my overflow tubes from the bottom of the carb. I ran them between the swingarm and the engine and they somehow got pinched. Maybe check this out on yours. Cliff.

[orkid1989]

As our esteemed friend TT would say; " When was the last 3,000 mile tune-up done ? ".

well i just bought the bike but i changed the spark plugs, wires and oil when i got it a month ago.

These bikes are to expensive to fool around with .You may ruin the engine. take it to a good Honda shop Before something bad happens to it. LOL az

Hopefully this guys just joking as what he is saying is BS. Much cheaper to do the work yourself than take it to a dealer. Anyway...what colour are your plugs - are they all the same colour? What size jets (pilot/main)? Have you set the timing? Valves? Sync the carbs after cleaning them?

Your engine is likely fine and more likely its fuel related/ignition related.  Got a manual? If so check basic tuneup section. Ask questions - photos help to diagnose as well.

Do yourself a favour and google "compression check motorcycle cold"


Good luck
Andy

ill have to take out the spark plugs tomorrow and check but the jets are stock and i have no way to check the timing (have no idea how) and not sure how to check the valves either but yes i did sync the carbs after cleaning them

ill have to check after the engine cools down. the carbs are freshly cleaned so i doubt that. could it possibly be the jets? right now it has stock jets with pod filters and a 4 to1 exhaust. also i checked the compression yesterday and it was 100psi so maybe thats where the problem is?

If you are 100% sure you have stock jetting and you have pods, the pods modification now allows the engine to pull in a lot more air but with stock jets the fuel flow is little changed to match that increase in air flow.  I think your plugs will not look light tan when you check them.  With the addition of a 4-into-1 you need bigger mains, probably bigger pilot jets too.  And maybe move the clip on the jet needle downward 1 notch.

(SYMPTOM: Overheating and low power)  +  (POSSIBLE CAUSE: stock jetting + pods + 4-into-1) = low power and overheating.

As someone said it sounds like a tough case and dangerous to ride the bike if it is that bad.

If you find time you can
- check points gap and timing
- check valve clearances
- check and probably change jetting

When you cleaned the carbs did you check/set the float level?

If you're getting 100psi *and* you had the throttle all the way open while you were kicking it, man that is kinda low compression, would think you'd burn oil.  I suspect when you did your compression test the throttle was closed...?

EDIT:  the shop manual showing how to check the stuff above is under 'Download Manuals' at the top of this forum or click here => http://forums.sohc4.net/index.php?topic=17788.0


Then scroll down a little and click on this to download the manual:

"CB750K CB750F Complete Service Manuals, Parts List Manuals & more 1969-1978 (HUGE FILE): http://www.mediafire.com/file/nzkmzodmgin/Honda CB750 1969 - 1978 Parts Lists - Service Manuals & More.pdf"

yes im sure it has the stock jetting because it has the new jets i got with my carb rebuild kits. im sure it needs a larger size jets but im not sure what size to get. i did check the gap on the points and it was good but not sure how to check timing and valves. also i did check the float levels when i cleaned the carbs and set them

[Spanner 1]

Repeat; ' 3,000 mile tune-up please'... lots of reading/understanding opportunity for the questioner is available here... or move towards a new(er) sportbike... old SOHC Honda's are not for everyone 

[mrbreeze]

What leads you to believe it is overheating? What is it doing?White spark plugs are a good giveaway. Does it sound like Rice Krispies when you shut it down after a ride? If it does...you are running lean and slowly destroying your engine. Give it the tune up that is mentioned all through this forum and don't skip any steps.You need more fuel or less air.You will have to experiment to get to a happy medium but once you're there....you won't be able to get the smile off of you face!!!Good luck with it.

[orkid1989]

well it wasnt exactly crackling but i could feel the heat coming off of it and it died when i came to a light after getting off the highway and was steaming. it wouldnt start up till it cooled off. im thinking it mostly needs to be rejetted but im not sure how to choose what size jets to get and how to tune it when i do. as far as a tuneup i did one myself with oil, plugs, and wires less than 50 miles ago so do i really have to do another?

[andy750]

750s run best after a full tune up:

1. Check timing
2. Check valves clearances (intake and exhaust)
3. Synch carbs after making sure your float heights are all at 26 mm.
4. Spark plugs are all good and firing well on all 4.

Without this you have no base and are shooting in the dark.

If you have changed anything from stock such as pods or exhaust rejetting will be in your future. I have a 1972 CB750 and to get the jetting right I went from mains 110 to 112.5 to 115 to 116 to 117.5 to finally 120s. Bike became happier and happier with each change. It takes time but its worth it when its all finished.

Plenty of more info on this forum about the tune-up procedure and as was said already get a manual (for free) from the download area.

good luck
Andy

[ekpent]

Just out of curiosity I have heard you mention wires twice in your tune up review. Since the wires are permanently attached to the coil what procedure did you do to change them? When you get in there to check those carbs and jets the mains should be at least a #120 and the slow jet a #40 since that is the baseline stock for a 71 engine. Always good to double check the serial numbers on engines when new to you also just to verify actual vintage.A static timeing would be a very good idea also,can do it with a 99 cent Harbor Freight electical probe.

[wrenchmuch]

Compression gauges vary in design . You can get different readings depending on the volume of air in the line ,and how much the line expands under pressure . Too Tired has posted on this before . Compression gauges are good for checking difference in compression between cylinders . To compare numbers to whats in the book you need the gauge honda used to generate the original numbers . A leak down tester is much better a diagnosing problems .
As stated before you need to do a complete tune up . Once you have the engine set up it will be easier to get the carbs set properly . Beware aftermarket carb kits . The  included jets usually don't meet the kind of tolerance standards the oem parts do . Especially the needles . Needles, main jets and ,pilot jets last a long time . Consider re-using these parts if they are in decent shape . Its hard to advise regarding jetting by computer . Here's a guide for reading plugs .
http://www.dansmc.com/spark_plugs/spark_plugs_catalog.html

This is a good ref. for jetting .

http://www.justkdx.dirtrider.net/printcarbtuning.html

[ekpent]

BIG +1 on wrenchmuch. I have seen aftermarket carb stuff that is total junk,just creates more problems. Clean the old well and re-use.

[andy750]

I wonder if the main jets are 110  -if an aftermarket jet kit from an online supplier there is a good chance its a generic one-size fits all kit....

[orkid1989]

ok. im going to have to check up on how to check  the timing and valves butim sure it cant be too hard. i know that i need jetting though because it has stock jets in it with pod filters and a 4 to 1 exhaust made by carpy. the thing im not sure about is what size jets to get. and sorry for specifing but i ment plug wires

[Hondawggie]

ok. im going to have to check up on how to check  the timing and valves butim sure it cant be too hard. i know that i need jetting though because it has stock jets in it with pod filters and a 4 to 1 exhaust made by carpy. the thing im not sure about is what size jets to get. and sorry for specifing but i ment plug wires

If you could remove all 4 spark plugs and comment on their appearance, the help offered will become alot more relevant -- it is easy to remove the 4 plugs, can you have a look at them then post what they look like?

[CrankyOldGuy]

Concerning Andy's remark concerning the generic 110 jets.  I ordered a carb rebuild kit for my K1 from DSS and received a Keyster KH-0134N carb rebuild kit.  The box said it was for Honda 750K0/K1/K2 (ears perked up at this) .  It contained a 110 main jet (K2) not 120 (K1).  It also said it was for original carb no. 16100-300-034 (K2) not 16100-300-024 (K1).  Confusing to say the least.  One has to be careful what they are looking for and actually receive.  Fortunately I was not looking for the jets.

Lesson - order jets separately and OEM if possible.

Harry O.

[ekpent]

So you are saying then that you DO have the #110 main in now,that is confirmed? Might want to start with a 120 and stay away from the "kits"

[orkid1989]

So you are saying then that you DO have the #110 main in now,that is confirmed? Might want to start with a 120 and stay away from the "kits"

it has the stock 120 but it also has a 4 to 1 exhaust and pod filters so im assuming that i will have to get a slightly larger one. granted i know almost nothing about bikes.

ok. im going to have to check up on how to check  the timing and valves butim sure it cant be too hard. i know that i need jetting though because it has stock jets in it with pod filters and a 4 to 1 exhaust made by carpy. the thing im not sure about is what size jets to get. and sorry for specifing but i ment plug wires

If you could remove all 4 spark plugs and comment on their appearance, the help offered will become alot more relevant -- it is easy to remove the 4 plugs, can you have a look at them then post what they look like?

i was wondering when would be the best way to check them? as in should i keep the rpm up for a while or just ride around town? also should i take them out immediately after riding it or let it cool down first?

[orkid1989]

if the main jet is a 120 like stock and the slow jet is a a 40 like stock then what should i modify in order to compinsate for the pod filters and 4 to 1 exhaust? i thought that the best way would have been getting larger jets. i found a website but im not sure how accurate the info is on jetting http://www.motorcyclecarbs.com/Motorcycle_Carburetor_Jetting__W7.cfm if it is then i need 128 main and a 42 slow jet. it sounds like it might be a bit off though

[ekpent]

Not all engines respond the same way to jetting changes. For you its the trial and entry stage.Depending on the overall miles and condition of the motor there can be different mixes in your recipe.Just make sure now for starters that you know your engine is a 71,in fine tune except carbs,and the parts in them are "real".Stock air boxes are cheap to get and may be a good option if you are going to ride it a lot in different environmental situations. Have we seen a pic of this beast yet   Might let us have a little more insight. Here is one of mine . Whoops 2 my bad

[nancy]

Jet size is likely OK - but needle may need to be moved. Standard position of 3 down from top is designed for standard specs as I understand it. If you change that clip position and lower or raise the jet needle,...you are changing the fuel volume entering and will make running leaner (needle is lowered/clip is raised) or richer (needle is raised/clip is dropped). I have just done this on my carbs - I dropped in 120 mains jets and found I was sooty on all carbs. So I pulle dthe carbs and changed the clip to 2 down,...lowering my needle....limiting my fuel intake. I now have greyish plugs.
MOOIMBW

Mark

[ekpent]

Mark you did good on really getting into them,the carbs. When you do a clip change read up as you are going deep into the belly of the beast. Be prepared to remove the slides and disassemble that section. Read up.

[orkid1989]

Not all engines respond the same way to jetting changes. For you its the trial and entry stage.Depending on the overall miles and condition of the motor there can be different mixes in your recipe.Just make sure now for starters that you know your engine is a 71,in fine tune except carbs,and the parts in them are "real".Stock air boxes are cheap to get and may be a good option if you are going to ride it a lot in different environmental situations. Have we seen a pic of this beast yet   Might let us have a little more insight. Here is one of mine . Whoops 2 my bad

well the motor has about 27000 miles and it has pod filters and 4 to 1 exhaust.  and it is the original 71 engine on the frame. also i tried to upload a pic of it but it says its too big.

Jet size is likely OK - but needle may need to be moved. Standard position of 3 down from top is designed for standard specs as I understand it. If you change that clip position and lower or raise the jet needle,...you are changing the fuel volume entering and will make running leaner (needle is lowered/clip is raised) or richer (needle is raised/clip is dropped). I have just done this on my carbs - I dropped in 120 mains jets and found I was sooty on all carbs. So I pulle dthe carbs and changed the clip to 2 down,...lowering my needle....limiting my fuel intake. I now have greyish plugs.
MOOIMBW

Mark

o ok so move the clip from 3 down to 4 down? it makes sense to me. i never thought of it before.

[nancy]

Yes Orkid - move clip 3 to 4. But that is my guess. It was (seemingly) a lucky one in my case. Worked for me. It may not be your salvation though. First and foremost - you must read the plugs and guage fuel and ignition issues from their appearance. Sooty or whitish.
As suggested - to change clips means removal of throttle slide (easy) followed by removal of the needle retaining bracket hidden up in the slide's bowels - this is either a snitch (mine were) - or a bastard. The screws may well be stuck harder than hell's backdoor. DON"T DON'T burr em with an ill-fitting screwdriver. Get the right fit,..get a good firm downward push on the handle and turn the sucker firmly. These screws if originals are put there to make your life miserable and money for the mechanic you go to to fix them up.
+1 on READing...B4 getting into bowels.
Regards
Mark

[Hondawggie]

If you could remove all 4 spark plugs and comment on their appearance, the help offered will become alot more relevant -- it is easy to remove the 4 plugs, can you have a look at them then post what they look like?

i was wondering when would be the best way to check them? as in should i keep the rpm up for a while or just ride around town? also should i take them out immediately after riding it or let it cool down first?

Well the technical term is 'doing a plug chop' but it will be helpful to see the plugs in their current state before you start changing jets.  Remove all 4 plugs and describe them or take 1 photo and upload it.

Before you start changing jets, these things would help:
- what do the plugs look like
- are the value adjustments in spec (I gave you a link to the manual earlier in this thread).
- what are the points gaps and is the timing set Okay (use the 'static' timing check as described in the manual, ask if you're not clear on that)

Here's the deal, if your plugs are sooty/very dark, re-jetting needs to take a backseat to finding out why you have your performance issues and really dark/sooty-looking plugs.

If your plugs are very clean and whitish, well yes re-jet may be in order (the pipe and pods are driving this issue) but..........gotta just unscrew those 4 plugs when the motor is cold and tell us what they look like.

EDIT:  Here again is the manual:
The shop manual showing how to check the stuff above is under 'Download Manuals' at the top of this forum or just click here => http://forums.sohc4.net/index.php?topic=17788.0
to get to the page with all the manuals you can download.

Then scroll down a little and click on this to download the manual:

"CB750K CB750F Complete Service Manuals, Parts List Manuals & more 1969-1978 (HUGE FILE): http://www.mediafire.com/file/nzkmzodmgin/Honda CB750 1969 - 1978 Parts Lists - Service Manuals & More.pdf"

[andy750]

also i tried to upload a pic of it but it says its too big.

You can use a photo sharing website like photobucket.com to link to your photos. Upload your photos to photobucket (or similar) and then copy the IMG link (next to the photo) and paste it in your message - this will show your photo in the message. Easy!

 

[wrenchmuch]

We want you to do the full tune up in order to eliminate all other possible causes for your problem so that you can focus on the carbs . If I were to make a guess at the main jet you need I'd say #125 or 130 and one step richer on the needle given the changes from stock. You could also be lean because of a intake leak or running too hot because you have too much ignition advance . Pods can make it  difficult to get things set right in my experience  . If you are after performance you should consider a stock air box with a K&N filter in it . If you like the way pods look that's a different story .
I email pics to myself and the file size gets smaller in the process . I'm not very computer savvy and this works well for me .
good luck
Mike

[Hondawggie]

Here's a really really easy way to shrink the size of an image.

1) take the photo with your digital camera

2) transfer it to your computer (this assumes Windows not Mac, not sure if this works on mac)

3) now with the photo saved as a file on your computer, open it

4) now hit "Ctrl-Alt-PrtSc"  or, if you have a laptop like mine, I have to hit "Fn-Alt-PrtSc"
    (the 'PrtSc' button, aka "Print Screen"is usually on the top row and is combined on the same key with another label like 'Insert' on my laptop.  This button takes a snapshot of the top window so make sure the window that is displaying your photograph is the topmost window on your computer screen)

Okay, after you do "Ctrl-Alt-PrtSc" you just took a 'snapshot' of the window that was on the top of all the others on your computer screen, and hopefully you put your photo window on the top  before hitting "Ctrl-Alt-PrtSc" (click on the title bar of the photo window to bring it to the top and/or move it a little, then don't click any other windows).

5) Now go to Start=>All Programs=>Accessories and click on the Windows 'Paint' program.  The Paint program window will launch.

6) Now hit "Ctrl-V" to 'paste' the screen snapshot you took of your photo in steps 1-4 above.  If it doesn't appear and you used "Ctrl-Alt-PrtSc", try "Ctrl-PrtSc", or "Fn-Alt-PrtSc" in step 4 above until you can get your photo to appear in 'Paint' by hitting Ctrl-V.

7) when you successfully paste your photo into the Paint program,  use the 'erase' or the selection rectangle in the Paint program to trim your photo in Paint so all you see is the photo and not the title bar, status bar, menu bar of the window that originally held your photo.  (This is for neatness and is optional.  Even if you don't do this clean up, your original photo file size is gonna shrink bigtime.)

Now select 'File->Save' in Paint to save your image to a new file.  I usually keep the same name as the photo's file but add "_smallFile" at the beginning to remind me it's a shrunken copy of an original photo file.

I use the above to shrink a 1 megabyte image to 100kbytes or less.

My camera made a .jpg file of size 1.42 megabytes for a side-shot photo of my bike.

Using the Paint program process above, it shrunk to 82.3K bytes.  Huge improvement.

I've been using this homemade file-shrinker procedure for a long time to upload photos to the web and to send them as attachments.  I'm sure there's other ways but the cool thing with this method is that every version of Windows has the 'Paint' program so you don't need special software to shrink images, you can do it on any computer running Windows.

To me the photos look identical.  I don't know the details of why my digital camera is so inefficient in the size of the file.

[lucky]

These bikes are to expensive to fool around with .You may ruin the engine. take it to a good Honda shop Before something bad happens to it. LOL az

A honda shop will not know anything about these bikes!!! Forget it.

[Spanner 1]

Zzzzzzz..... another Lucky 3 yr. old thread... with all respondents D.O.A. for YEARS..... GO Lucky !!

[orkid1989]

Haha he is right though. My local dealer wont touch it. I think they wont work on anything older than 1980.

[Stev-o]

Another one risen from the dead! Go Lucky!!