Robert Frere
CB750 Member
Hi I have a 1978 HONDA CB 750F2 my problem is that it struggles to get past 4000rpm . it's completely standard apart from the pistons that are836cc wiseco
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Poor running over4000revs
- Thread starter Robert Frere
- Start date
Have you checked ignition timing? ignition advance? Point gap and dwell?, Plug condition and gap? valve clearances? Compression? Carbs clean? Float levels? What main jet size? What air filter setup? Exhaust setup? If its stock jetting with the 836 then it is lean. Answer these questions then Ill go from there.
Funny but true. I adjusted my valve shims and timing (and idle), and vacuum sync before she ran above 5-6k RPMs. There are a lot of variables that go into your issue and you need to start narrowing things down.
Qaxe
CB750 Addict
"Have you checked..." is the story of my motorcycling life. I need a switchboard with lights on it under my seat...
Robert Frere
CB750 Member
Standard airbox.exhaust.carbs have been sonic cleaned.float levels are exactly to spec new plugs no1sooty outer darkish brown eletrode.no2 not as sooty and electrode a bit lighter than no1.3is same as 2 4 is a bit leaner.have adjusted the air screw's accordingly.points and condensors are brand new genuine Honda in a brand new backplate. Main jets are 105standard tappets are adjusted as per the workshop manual.
Qaxe
CB750 Addict
BrettP, do you suggest spraying a quick blast of carb cleaner on the boots while the bike is running for that, or something else?
Robert Frere
CB750 Member
Theinsulaters to the engine are brand new as are the airfilter boots
Robert Frere
CB750 Member
Everything you mentioned has been done all tappets are good.vac sinc is spot on with the pistons and bores being new compression is superb it doesn't smoke it doesn't rattle . would raising the needles one notch help and is it possible to do that without completely strippng the carbs
Yes, that's what I do. As do many others. Top, bottom, and sides of the boots, front to back.
Robert Frere
CB750 Member
Its choking and bogging down but if I just back off the throttle a very small amount it's just settles a bit as for the adjustable needles I'm not sure it has them just someone suggested it I don't remember seeing them but I'll check
Qaxe
CB750 Addict
Just a couple thoughts.
Is the fuel filter new/clean? This causes "bogginess" in more than just motorbikes.
- The adjustable needles on the top of your carbs are definitely accessible without taking the carbs apart. Each carb. has a rubber dust cover hiding the tops of the adjustor nut and lock nut.
Personal note: It's awfully tight in there, so if you have to "go there" you might want to raise the back of the tank and rest it on a small block of wood, etc. to create some space for working.
Is the fuel filter new/clean? This causes "bogginess" in more than just motorbikes.
- The adjustable needles on the top of your carbs are definitely accessible without taking the carbs apart. Each carb. has a rubber dust cover hiding the tops of the adjustor nut and lock nut.
Personal note: It's awfully tight in there, so if you have to "go there" you might want to raise the back of the tank and rest it on a small block of wood, etc. to create some space for working.
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78 doesnt have adjustable needles...you have to shim them
sohc motor specs .002" intake and .003" exhaust
Did you use the F2 specific pistons? If you used K pistons you will have a lower compression ratio.
Being a F2 with 836 I would start with at least a 110 main jet, maybe a 115.
First thing I would do is go ride it, when it hits the wall at 4000 rpm, hold the throttle there and start closing the choke....if it starts to pick up then its lean if it boggs down worse then its already rich but I dont see that being an issue.
I start new builds with 1 or 2 jet sizes bigger, then ride it. Take it for a good run and hold wide open throttle if it will do it and start closing the choke to see if its rich or lean then adjust and try again till wide open is good then you have to start playing with the part, half and 3/4 throttle which is when you will start shimming the needles.
Digger, doesn't the bike need .005" valve clearance? Won't .002" burn the valves?
Not on the sohc completely different engine design than the dohc
Robert Frere
CB750 Member
Sorry but there's no rubber dust cover on my carbs there is an aluminium top held by two screws.the fuel tank is brand new with the pet tap
Robert Frere
CB750 Member
Also just had the bike running in the yard and it will rev up to 5000rpm but seeing as it's got new pistons and bores didn't want to put to much strain on it but it reved cleanly but it wasn't the same as riding it.will take it out ASAP
