Topic: 71 cb750 engine problems

[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!!