Engine won’t idle or rev above 4000 after camshaft replacement

StobbJobb

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Hello!
I just finished replacing the exhaust camshaft on my 1979 CB750L and now I’m having issues where the bike won’t idle or stay running without the choke lever pulled fully out and doesn’t seem to rev above 4000.
I ran the bike before doing the replacement and it started up great (forgot to pull the choke out, but it fired right up) and stayed running until I hit the killswitch.

When I pulled the cover and put it on “T”, I noticed that the notch on the end of the intake camshaft was one tooth behind the exhaust camshaft. I corrected this issue and now the notches on the ends of both camshafts line up the same.

Initially, I thought it might be the slight difference in lobe size on the “new” exhaust camshaft and change in valve clearance. Then I thought it might be a fuel delivery issue, but I just cleaned and rebuilt the carbs, so probably not that. Then I started thinking about the adjustment I made on the intake camshaft and how that could have made significant difference in my timing.
I’m leaning towards a timing issue, since the bike started right up and revved strong before the adjustment.

I am looking for some feedback on where to start digging, before purchasing a bunch of tools/parts I don’t need to diagnose/correct my issue. I have ordered a compression test kit, but it won’t be here for a couple of days. I am trying to get my timing light back from a buddy, but I don’t have it yet.
I’m new to motorcycles and motorcycle troubleshooting, but in general, I would say I’m mechanically inclined and intuitive. Any feedback or suggestions would be greatly appreciated.

Thanks
Matt
 

amc49

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You don't say what the original cam got changed for, and incorrect work if you didn't reshim the valves for the new cam, they are not the same and never are.

You do NOT use the notches on ends of cams to time cams with! Only the center sprocket marks. The end-of-cam notches change position to use a different spot to set valves in on different years, Honda not recognizing that the valve clearance number they use was wrong, should be .005" instead of .003" as a go to number.

Original chains will be stretched and the marks then must drag slightly behind, the choice lots of times ends up being one of the sprocket being a 1/2 tooth off either way, you cannot get it dead on. You put it to where the cams are slightly retarded and correct. Front of sprocket mark will likely be slightly above the head line and back of sprocket mark will be below it and both sprockets roughly the same.
 
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StobbJobb

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Thanks for the response.
The original camshaft was damaged after replacing the oil seal on the tach drive holder and upon reinstall, the gears of the tach drive/camshaft were not properly meshed together.
I did not reshim the valves. Cam lobes measured .001-.003 different between camshafts.

When I removed cover and noticed the end-of-cam notches weren’t aligned, I looked at the position of the lobes on No. 1 of the intake and exhaust and saw that they were not in line with each other. I did not note the sprocket punch marks on the intake camshaft before removing it.
When rotating the crankshaft by hand, the chain would be taut, then go slack for a moment before tightening back up.

When reinstalling the camshafts, I did use the sprocket marks on both camshafts in accordance with factory manual to time the cams.
After reinstall and chain adjustment, when rotating the crankshaft manually, the chain stayed taut all the way around.

Here are a couple links to images before disassembly. Same position, different angles. One shows the end-of-cam notches and the other shows the lobes of No.4.
https://drive.google.com/file/d/1QQjEnWXbEt3UZqpoCIbbTZNgCTahBbmi/view?usp=drivesdk
https://drive.google.com/file/d/1V22q3RpLSOioEs8ohPJLY6LCk5JdH7Uc
 

amc49

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'Cam lobes measured .001-.003 different between camshafts.'

That right there alone can kill you. The service manual is wrong on clearances as .002" can and WILL burn valves, don't ask me how I know that. .002" of every measurement gets tossed out as it is not real when the engine is running. That means a carefully measured .003" is only .001" in reality. Why you seek to .005", a .004"-.006" range works really well.

The compression test should show it, they commonly are low on these.
 
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