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Anyone know of a reputable DOHC mechanic in NJ? Can't Figure my bike out.

Markuli

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Hey everyone,

New 1979 CB750k owner here, and the bike is great but it definitely isn't perfect. The main issues I have noticed, after going through it to the best of my ability, are:

-Three cylinders run lean based on looking at the plugs and seeing a white tip and no fuel residue on the faces, but cylinder 2 has (what looks to be) dry fouling, we're talking solid black color. I don't have pics at the moment because I'm on vacation. Same results from replacing spark plugs twice.

-The bike sputters upon acceleration, and when holding at around 5000 RPM there seems to be some sputtering as well.

What I've done so far, in chronological order:

-Checked compression with crappy harbor freight tester. With all plugs out and WOT throttle, I got 90psi across the board. As I said it's a crappy gauge, and my bike starts easy and runs so it's probably reading low.

-Cleaned the carbs, blew brake cleaner and compressed air through every orifice I could find (and shot an idle jet plug into low orbit in the process), replaced two of those idle jet plugs which didn't seat snugly.

-Installed a new battery as the old one didn't hold a charge for more than two days. Now, it's smaller than the old battery, but it's a 12v and reliably starts my bike. "maintenance free" lead acid battery.

-Replaced all coils, wires, boots and plugs with new production replacement parts. Originally the bike didn't rev past 5000rpm, it now can get to 9000 after replacing everything.

-Synced the carbs with that 60 dollar quad gauge off Amazon.

-Checked my valve clearances, all my intakes were at .003/.004, and all my exhausts were at .005" with one at .006" on cylinder 2. I got all my intakes to .005" with the exception of 2 at .004, I can't currently remember which ones stayed at .004 right now.

-Checked on my pulse generator, the clearance between the lobe and pickups was out of spec at around .040", brought both down to .016" I didn't check the resistance on it though.

And that's where we are today. I had just done the pulse generator Thursday, and now I'm on vacation so I have some time to write up this post!

To see if it was a carb issue, I pulled the choke out during acceleration and didn't notice any change in the bike's behavior. At idle the choke does its job, so I know it's working.

To see if I had a dead cylinder or something even with the uniform compression, I pulled each boot off its plug at idle and noticed a difference in idle sound with every cylinder.

I really don't know how much power I should be expecting from the bike, but from what I've read on the forum it seems like the bike isn't delivering all it's got. I've pretty much exhausted my mechanical capabilities up to this point, which is why I am asking for recommendations for a guy in NJ who really knows DOHC's to put a fresh set of eyes on my project. But I'm willing to try some more stuff if you guys think I can do it myself, based on what I've done already.

Thanks in advance for anyone trying to help me figure out this bike's issues. I really do enjoy riding it, but I know it has more to offer and I want to get this bike running how it did off the factory line in 1979 (or at least kinda close!)
 
You need a more reliable compression test. 90 psi should not even hardly run and should foul all 4 plugs.

Something wrong seriously if you can choke at acceleration and zero effect, choke affects more the higher you load the engine. Should kill a proper running bike instantly.

Cylinder two plug being black has something wrong there, weak ignition or compression or carb issue.
 
Finally got around to borrowing another compression tester to use. I forget who makes it but it was a mechanic buddy's kit. It had two gauges in it, one for car engines and one labeled "small engine gauge". The large engine gauge would pump up to 75 psi in two cycles then stop moving at all, like it didn't even flutter.

So then I tried the small tester and voila, 140psi across all four cylinders.

Knowing that I didn't have a dead engine, I sent it down to a local guy who works on vintage bikes. He's gonna go through the carbs again and keep me updated, which I will then relay to this thread for anyone else who gets these symptoms.
 
Also I did it using a car battery (car was off). The car didn't even notice it when I started it back up. Much better than stressing the puny motorcycle batteries!
 
I wonder if a broken valve spring or valve train damage would cause such problems. It looks like you've checked nearly all culprits. How about crap in your gas tank? Any rust?
 
Mileage?

With more the chains stretch and then cams retard to cost some power. Mileage also means valve seals harden, where your one plug may be going black from if not a fuel rich issue. The seals can cook and leak oil.
 
Valve springs are all good. The tank seems to be rust- free but I know that particles the human eye can't even see will clog up carbs. Mileage is 16,000 and change right now, it's possible a valve seal is bad. Right now my mechanic is going through the carbs to start, it's entirely possible that I just didn't clean them well enough.

Question on cam tension: How much play should there be in the chain? Like if I were looking at the top of the chain between the two cams with the cover off, how much droop should there be?
 
There is no droop at all, the chain tensioned right will be tight. Not super tight but tight nonetheless. No slack permitted.

HOWEVER, there is forced 'droop' if you will from the tensioners themselves, they put the chain in a curve but the curve stays tight too. The spring loading on the tensioners keep the chain taut, they die quick with any 'loose' there at all.
 
UPDATE TIME!

I had the bike over at the mechanic for a while, mainly due to the fact that I haven't had time to come get it from him.
He has gone through the carbs and synced them as best he could. The lean problem is gone. But, while the bike runs better, the sputtering problem has not gone away. It still sputters and fights acceleration, seemingly from cylinder 2. When I pulled in from my 50- mile ride to my house, the left exhaust smoked when I twisted the throttle a bit.

My mechanic also did a compression and leakdown test and said the results were as perfect as a 40 year old bike can muster, just like my findings. He thinks it's either the carbs just being beat/worn out, or an ignition problem. The funny thing is, that he thinks my ignition pickups have been replaced at some point, but they seem like the oem ones.

Even though I replaced the coils, plug wires and boots, I still feel like the problem is somewhere in the ignition system. I am by no means an expert, but there's nothing about the carbs that really leads me to believe they're worn out, especially at a mere 16,000 miles... Could it be that I broke a spark plug wire twisting a boot into it/ the wire into a coil? (Side note- I hate that design)

To answer Wez, here's pics of my gas tank, doesn't look like the kind of rust that causes problems. Also here's a pic of the fuel sleeve from by borescope.
1003172025.jpg1003172025a.jpg1003172022.jpg
 
FYI the fuel sleeve generally NEVER stops the fine rust if they run ethanol in your fuel, it goes right through that filter, the holes are hugely too big.
 
Is that fine rust problematic? I feel like my mechanic would have picked that up when going through the carbs.

Could my air filter be causing any issues? I found that a mouse or something had stuffed seeds in the folds at one point, but I blew them all out with compressed air before putting it back in.
 
Ah ok. Good to know, but as of now the bike isn't flooding, so i do not suspect rust to be the culprit. Also since it's spitting out what I assume to be unburnt fuel, it must be getting airflow in that cylinder to bring fuel in, meaning my airbox isn't blocked anywhere.
 
So as I said earlier, the lean problem had been solved by the mechanic, but cylinder 2 was still missing and has what I would say is a rich misfire. I pulled the carbs off again, and lo and behold, I'm missing a needle jet on none other than cylinder 2. I didn't notice that the first time I had the carbs out in July, as they don't come out when you unscrew the other slow jet piece. Looks like I need to source one of those down.

I found a set of the jets (the collars, not the needles) from a cb750F online, does anyone know if they're interchangeable or is the taper different from a '79 750k to a '80 750F?
 
They should swap.

Not tapered, the holesize is the restriction there, the needle is what tapers to vary that restriction. You are confusing parts, that is the secondary main jet tunnel there, the slow jet is elsewhere. The part can easily fall out when the jet holder is removed, they simply slip into place although sometimes they get stuck.
 
Ah ok. I thought the needle jet (collar) had a taper along with the needle. From google it seems that the top half of the jet is just wider than the bottom half. Either way, one claiming to be from a 79 750k from Ebay is in the mail and should be here Friday, so with my luck I'll probably get it next Monday...

Well whenever it shows up, I'll update with how she runs.
 
BIKE'S FIXED!

Turns out that, a part which is supposed to restrict fuel flow, not existing in your carb, can cause a rich condition in your engine! Who woulda thunk it. Now the carbs need to get synced again, a fresh set of shoes put on, and maybe install those CBR600 F3 coils I bought on a whim. Bike runs strong even with out-of-sync carbs. Lightning fast throttle response.

For anyone who installed CBR600 coils, how did you make your mounting bracket? Just looking for ideas.
 
With all due respect............drive a DOHC bike with direct lift carbs on it, CV carbs are slow, oh so slow............one whopping reason why CVs are used, it saves lots of 'sudden acceleration' lawsuits against the OEMs. Why they were designed.
 
It's all about perspective. Compared to what it was like when I had a dead cylinder, the response is night-and-day. But compare the working CVs on this bike to a FI bike and, once more, night-and-day difference. Lol
 
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