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?