Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: royhall on March 13, 2015, 08:13:25 AM
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Here we go again with the dreaded F2 carbs.
Just re-furbed a set with as many new parts as are available. Carbs themselves are perfect, problem is now the accelerator pump.
It has a new diaphragm kit fitted and the non return valves are all clean. If I press the pump stem with a screwdriver they pump petrol perfectly, and the pump lever arm is free to move. But when I open the throttle its like the spring that pulls the lever arm across is not strong enough to operate the pump. So no petrol squirt.
Any ideas anybody?
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The throttle return spring is pretty strong so difficult to replicate the normal operation when you quickly open throttle.
Suggest fitting the throttle cable and trying with the throttle control.
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I hate F2 carbs they are terrible to set up
it sounds like you need a stronger spring on the lever arm or a softer one under the diaphragm
Pete
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Yep, I think you may be right about the bottom spring Pete.
Am mid changing diaphragm and spring back to the original one that was previously okay. Now waiting for another packing set as the Cruzimage float bowl seal has decided that now its seen petrol its going to grow by 10% so it can never be put back into the groove.
Hate those Cruzimage kits and come to think of it, the whole F2 carb setup as well. If I keep the bike I think I will convert it to K series carbs. :-\
"O for the simple things in life." ;D
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I have a Rickman cb750 with the F2 engine, last week i stripped the carbs and attempted to get the thing going, i had the carbs off over 6 times and was getting really pissed off, but i won the battle in the end and had the bike running and ticking over, thats a first for me to get a decent tick over with f2 carbs, now i have to replace the valve guides and seals, another bloody f2 thing,
I have heard that enlarging the pilot jets in these carbs can cure lots of the rough running on start up, but i cant finf the link to the post where i read it,
In theory sorting out the lean running on these should make the bike a whole lot better
pete
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Personally I think Trigger is right. Throw the bloody things away and retro fit F1 carbs. Think that may be my route for the future.
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Had the same experience working on a set of cb750 hondamatic carbs. Time isn't kind to the accelerator circuit on these designs. Problem with the Hondamatic is that you needed the enrichment circuit to be able to pull away effectively with the Hondamatic transmission. Finally got there but was a serious ball ache...If you can, fit the K1-6 carbs..
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Are their any re-jetting considerations when fitting K series carbs, as the heads on the F2 have different size valves.
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Stick with the standard jetting for an F1, 105mains, bigger valves just let the engine breathe more freely at high revs, and give a lumpier idle than standard K/F1 valves.
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Roy, if you need to borrow a set of K carbs to try then let me know...
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Thanks James that's very kind of you.
The reason I'm asking about fitting the older carbs, is that Trigger mentioned F1 carbs and I have a good spare set in the garage. For now though I'm going to stick with the standards as it's become a bit of a personal challenge.
I think the "I will not be beaten" mentality is shared by most people on this site. 8)
Cheers.
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:-) no worries - I'm sure you'll get there eventually...
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Just tested mine and not working :(
Have you seen this?
http://www.hondachopper.com/garage/carb_info/accelpump_valve_repair/Repairing_77-78_Accelerator_Pump_Valves.pdf (http://www.hondachopper.com/garage/carb_info/accelpump_valve_repair/Repairing_77-78_Accelerator_Pump_Valves.pdf)
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Got mine working a treat. It was all in setting the clearance from lever arm to stem. Needs to be very accurate, even slightly out and the bloody awful thing stops working.
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Mine has a blocked jet on no.2
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If My memory recalls me right they were were a git from new never mind 30 plus on
Cheers
Bitsa
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Mine has a blocked jet on no.2
Mine was the same, I sprayed carb cleaner on it in situ by bending the aerosol applicator tube through 180 degrees and surprisingly it worked!, must have had congealed fuel at the outlet, yours could be blocked prior to that but worth a try.
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Mine has a blocked jet on no.2
Mine was the same, I sprayed carb cleaner on it in situ by bending the aerosol applicator tube through 180 degrees and surprisingly it worked!, must have had congealed fuel at the outlet, yours could be blocked prior to that but worth a try.
I have flow from bowl to other jets, but not out of no.2, just been trying with carb cleaner and air line.
May try a strand of copper wire from engine side.