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Hard Starting , Popping, Missing Etc / 80 CB750F

mrmach5

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I recently acquired a very nice low miles 80 CB750F.
The previous owner acquired it after it had sat for a long time. He overhauled the brake system, new cables, chain, filters , checked valve lash, new cam cover gasket etc.
He had the Carb's professionally cleaned and new gaskets, accelerator pumps and air bypass valves. Also new manifold boots.
Probably need to be sync'ed

it is very hard starting,,,,, cranks and cranks then some struggles to come to life, stalls a couple times then finally keep it running, and start to take the choke off, it pop's and runs poorly and no power.
Plugs are soot black. Checked the timing and its on.

Obviously its running super rich, So where to start? ignition, or carbs?
Any insight would be appreciated.
 
Now you know why the words 'professionally cleaned' mean literally nothing.....................In fact, that is a call to arms mark of butchery at my house.

I wouldn't trust somebody elses' valve check for spit either. Not on these as most have not a clue how to do it.

I'd bet a million the carbs are messed up. Plugs a problem too, now they are good and fouled, they will not be starting up right even if carbs are right.

If battery not new after the sit then wasting time as well.
 
Yep you are correct, no knowing who worked on the carbs or what they did, I'm right back to square one. I tried new plugs to see if they would make a difference,,,, none. Battery is brand new.
Perhaps I will start with a compression test, and look at spark strength, before chasing carb issues.
 
You DID charge the new battery?...............one never knows..........looking for 170 psi compression. Forget book specs on valves; set at .005" if you want them to last. They can burn at the book figures. .002" is effectively zero clearance real world and valve leaks to burn.

You got the model with pressed pilots, almost impossible to clean the bleed air crossports above the idle feed restriction in the pilot. The actual pilot center opening can be cleaned with like 'E' string of guitar, or .013" wire diameter.
 
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Also, check your primary and secondary main jets. I cleaned my carbs and I noticed that in two of the carbs somebody had put the primary and secondary jets backwards. And 3 of the accelerator pump nozzles were clogged and the E string worked perfectly.


 
Ok,
some updates, first off just for backround, this 80 CB750F only has 3,900 miles on it (not that that means a thing)

First tested for spark off both coils , Plenty of spark , no problem there (I was hoping this was spark as this would have been the easy fix)

New plugs the from the last start up , approx. 4 minutes running time,,,,, 1 -3-4 all rich fouled already, #2 perfectly clean.

Compression test, cold, all plugs out, kill switch off, wide open throttle. #1- 160 #2-120 #3-160 #4-160

So I guess my first chore will be pull the cover and check the valve adjustment, and hope that #2 is out of spec and can get the compression back.

I would assume that there still would be a carb issue, but one thing at a time. Any tips on the air box R&R?
Thanks
 
BTDT, mine burned 2 valves at 5K miles. Ridiculous. Where I learned to loosen them up. They were checked twice by Honda techs before they burned of course, in hindsight it's clear the techs did not REALLY check them, like so many thousands of other bikes. The head design lends to crap results reading clearances anyway.

Go to setting valves at .005", the engine will love you for it. The book figure of .003" means you may be at .002" really and they DEFINITELY burn there. Why? The cam cap clearance is more than the valve clearance, your perceived number is not what it appears even if you measure 30 times. One clue is that the number will move all over the place by simply pulling cams off the mark and putting them back dead on again, you get different number every time. What you get static is not what the engine sees at running because the valvesprings push the cams around in the looser caps to change the valve number. Been through this on a lot of these. You loosen slightly to ensure the valves are always closed, the engine then runs very well and for long periods. The valves were heat-treated for low lead fuel, modern zero lead fuel is harder on them and they tend to recede into seat more than get loose and why so many close up to burn. The setting them closer only makes it worse.
 
Ok Thanks AMC for the help,
I'll get to work on the valve adjustment. I've never done it on a DOHC Honda before, a bit different than my Bonneville , where after measuring you remove the cams to access the lifters.
Looking over the manual looks pretty cut and dry, however looks like a I need that Honda valve lifter holder tool. I assume these are still available?
Best source for shims?
 
K&L Supply I think still sells shims, you can web search for them too.

Do NOT turn the motor with a shim missing. You can juggle them from tappet to tappet to lower cost sometimes. Keep the printed number face down so wear doesn't rub it off.
 
Just received the tool, but will be next week before I can start the process. Question, do I need to mic the shims or are the numbers stamped on them accurate?
Thanks
 
Always mic them, although they rarely wear they can under unusual conditions and those very few I do NOT use. I lay caliper jaws on them and look for light bleeding at the flats. Rarely you get wear at one if a cam lobe was going south on it.
 
Well I finally got the time to get it apart and measured up,,,,, the good news is it looks like the low compression cylinder #2 did have both Intake and exhaust valves way out of spec and likely the cause as to #2's lack of squeeze. You were correct AMC! So much for the previous owner telling me he did a valve adjustment. Can't believe these go so out of wack in 3,900 miles.
OK here are my results,,,,, any advice or thoughts as to which ones to leave alone would be appreciated.

Exhaust
#1 #2 #3 #4
O O O O O O O O
.004 / 268 .005 /270 .003 / 273 .003 / 270 .004 /270 .004 / 270 .006 / 279 .005 / 281

Intake
#! #2 #3 #4
O O O O O O O O
.003/285 .004 / 282 .002 /270 .002 / 270 .007 /279 .006 / 278 .007 / 268 .006 / 268

The intakes on #2 could be even tighter than .002,,,,, smallest feeler I had was a .003 and it would not go.
So now comes the chore of figuring which shims can be moved around to get back in spec, then what shims I need to get ordered up.
 
4 can run, prefer 5. For sure fix the 3's.

Once you are within running spec then take bike out and run the living crap out of it, often the new numbers settle in even more to have a couple more then out of spec again. It happens if carbon built up on seat and the hard running then cracks that up to let valve settle in better but then too tight again. After that second recheck you're usually pretty good for a while. Pain in the -ss ain't it?

Just as an example, I got another 3K mile engine after I burned the valves on the first one and that last one I reset all to .005", bike then ran to 24K added miles more before they needed work again. Get them right at slightly loose and it lasts a long time. Set to service manual average they commonly give running issues before 15K miles. That .002" closer really kills you.
 
Another thing that makes this a task is that shims , from what I have seen so far are only sold in .005 increments,,,,,, What the???????????
So far have checked Z1 Enterprises, and bike bandit.
 
Finding some in between sizes now on E-bay. Also looks like the Yamaha motor then run in the Legends Cars use the same 25mm shim. Lots of parts suppliers for them in the Charlotte area, I'll smoke it out.
 
So, question ,,,,, I'm finding some of the odd size shims but in some cases they will not get me to the optimal.005, so .006 is probably better than .004 due to their tendency to tighten up rather than loosen?
 
Yep. The steps of 5 are NOT .005", those are metric numbers jack. I'd correct 7, forgive me for not looking at them completely. The shims are in .05 metric increments, that's two thousandths difference in each step.

.10 mm. = .004" Americansky
.05 mm. = .002" Americansky
 
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