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Author Topic: Carb Rejet After Cam Change (F1 engine with K-model cam)  (Read 1255 times)

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Offline Mr. Clutch

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Carb Rejet After Cam Change (F1 engine with K-model cam)
« on: June 23, 2017, 07:14:15 AM »
Hello all.

I just wrapped up rebuilding the engine of my 1976 CB750F1 after catastrophic top end damage due to oil starvation. I sourced a replacement camshaft that I believe is from a K-model judging by the plain cam chain sprocket it came with as opposed to the lightened Super Sport sprocket. The new cam has a casting number of R11, if that means anything to anyone, while the Super Sport cam I took out had a casting number of R1.

It starts, runs, and drives well at low throttle openings but stutters and looses power starting at around 4,000 RPM and up with medium throttle to WOT. I think it is running a little lean. Does that sound right?

Apart from a period megaphone muffler the bike is stock. It has '76 F1 064A Carbs with the stock jetting of 105 mains and 40 slows.

My question is, since I now have what I suspect to be a K-model camshaft in my engine, would I simply need to rejet to K-model spec? So 120 mains as a starting point?
« Last Edit: June 23, 2017, 07:18:04 AM by Mr. Clutch »
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Offline flybox1

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Re: Carb Rejet After Cam Change (F1 engine with K-model cam)
« Reply #1 on: June 23, 2017, 07:19:13 AM »
No.
Pay attention to your idle mixture (tan plugs) and you should be good.
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

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Offline Mr. Clutch

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Re: Carb Rejet After Cam Change (F1 engine with K-model cam)
« Reply #2 on: June 23, 2017, 07:53:52 AM »
Idle is OK. All 4 plugs are tan.

It's the mid to high range of throttle opening (maybe 15% and up) and over 4,000 RPM that's giving me the issue. It pulled just fine before the rebuild. I'm not sure what happened.
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"Whether you think you can, or you think you can't, you're right." - Henry Ford
"Failure is simply the opportunity to begin again, this time more intelligently." - Henry Ford
"Don't find fault, find a remedy." - Henry Ford

Offline flybox1

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Re: Carb Rejet After Cam Change (F1 engine with K-model cam)
« Reply #3 on: June 23, 2017, 07:58:35 AM »
Quote from: Mr. Clutch on June 23, 2017, 07:53:52 AM
Idle is OK. All 4 plugs are tan.

It's the mid to high range of throttle opening (maybe 15% and up) and over 4,000 RPM that's giving me the issue. It pulled just fine before the rebuild. I'm not sure what happened.
RPM is irrelevant.
You could be in 5th gear at 4000rpm and be running off the pilot jet, or in 1st gear at 4000rpm and be pulling thru the main/needle circuit.

So, think of it in throttle positions.
What throttle position are you finding the stumble? 1/4? 1/2? 2/3? 

You'll need to perform a proper plug chop at this throttle position, and have a look at the plugs.
With that information, you'll know what to change and how much.
« Last Edit: June 23, 2017, 08:21:55 AM by flybox1 »
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'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline Jimray23

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Re: Carb Rejet After Cam Change (F1 engine with K-model cam)
« Reply #4 on: June 23, 2017, 09:15:19 AM »
What about your jet needles, where r they at? Make sure they are clean with no obstructions or serious wear.

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Jim
I really don't see why people ask for advice if their not going to take the advice given.

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Offline Jerry Rxman Griffin aka MuthaF'er

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Re: Carb Rejet After Cam Change (F1 engine with K-model cam)
« Reply #5 on: June 23, 2017, 09:26:05 AM »
The F0/F1 used a different part number for the needles/needle jets. This would suggest the needles have a different profile probably due to the increased compression and the different cam profile.
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Offline TwoTired

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Re: Carb Rejet After Cam Change (F1 engine with K-model cam)
« Reply #6 on: June 23, 2017, 10:42:08 AM »
Quote from: Jerry Rxman Griffin aka MuthaF'er on June 23, 2017, 09:26:05 AM
The F0/F1 used a different part number for the needles/needle jets. This would suggest the needles have a different profile probably due to the increased compression and the different cam profile.
Perhaps even more significant is the higher pressure muffler.

If the carbs are tuned for the expectant less scavenging of the high pressure stock muffler, they could be adjusted for leaner mixture delivery.   An open exhaust would then result in a lean mixture without carb modifications.

We'd need a W.O.T plug deposit inspection to know if mixtures are indeed lean there.

Cheers,
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Offline robvangulik

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Re: Carb Rejet After Cam Change (F1 engine with K-model cam)
« Reply #7 on: June 23, 2017, 11:31:59 AM »
If I were Mr.Clutch (I know, I am not) I'd simply put in larger mainjets and see what happens. With these carbs that is far less work than "chopping plugs" on a bike that doesn't run right anyway. The only tool needed is a smal screwdriver to get jets out an in again, and going richer poses no danger at all. It can only run (even) worse, but that is not very likely, is it? ;)
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Offline Mr. Clutch

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Re: Carb Rejet After Cam Change (F1 engine with K-model cam)
« Reply #8 on: June 25, 2017, 12:21:35 PM »
Quote
If I were Mr.Clutch (I know, I am not) I'd simply put in larger mainjets and see what happens.

I ordered larger main jets. 120's, actually. Maybe a bit overkill but that is what I believe most pre-'77 K engines should run. They should arrive this week.

One other thing I noticed. It's a longshot but I really don't know. When I reinstalled the 4-1 exhaust system (with new copper crush gaskets) I noticed the finned flange does not go all the way to the machined surface on the head. There is a 3-5mm gap between all four flanges and the head.

Is there supposed to be a gap? If the flanges aren't tightened down enough an exhaust leak at the header could be to blame.
« Last Edit: June 25, 2017, 12:27:29 PM by Mr. Clutch »
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"Whether you think you can, or you think you can't, you're right." - Henry Ford
"Failure is simply the opportunity to begin again, this time more intelligently." - Henry Ford
"Don't find fault, find a remedy." - Henry Ford
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