Hello everyone!

A small problem with my newly aquired steed! It misfires under load until you hit 4k revs, then it tears off. The problem is remedied to some extent by pulling out the choke. Not as clean a pull but much better running.

My guess is that a different carb jet comes into play around 4k revs hence it sorts the problem. It starts and idles well.

However, its very undriveable below 4k revs, and its a ball ache to keep pulling the choke in and out.

Any recommendations? Im going to take it to a rolling road, ive heard the boggs brothers mentioned, is this their sort of thing? Need someone who is a weber expert really.

Much appreciated for pointers

Joel, what size cross-flow, and what jets have you in there? What fuel pump have you got and have you got a pressure regulator in the system? Can't really guess a diagnosis until I know!

Fozzie

Webers have two sets of jets; idles and mains and a set of progression holes to over the transition between the two. In addition to this the carbs also have an accelerator pump jet for each barrel which squirts a jet of fuel each time the accelerator is depressed to keep the mixture rich under rapid throttle opening conditions). The main jets start to operate at about 2500 - 3000 rpm. It sounds like either your idle jets are too small (engine running lean) hence the misfire and the choke curing the problem or an issue with the progression holes (the idle jet feeds the progression circuit so again too smaller idle jets would affect this as well). Other problems with the progression could be
;
1. too few progression holes (probably unlikely)

2. The butterfly in the wrong position in relation to the first progression hole - with the engine idling the butterfly should be just about to uncover the first progression hole.

3. Blocked idle jets or some other restriction to fuel flow in the idle or progression circuits.

i'm not expert but I have learned loads from reading documents on the web and "tinkering" with a set of DCOE 45's.

attachment below may help

[Edited on 31/7/05 by givemethebighammer]

Do you know what cam you have in there? Lots of overlap may give problems at lower revs.

Could the timing be slightly out?

cheers all.

Its been bored out to 1640cc, and i thnk its got a fast road cam. Not a clue about jet size! Seeing as i cant check most of these things myself, i guess i'll have to leave it to the experts... ah well.

Cheers again!

good link that though, GMTBH. Most interesting!

what size idle jets ?
what size venturis ?

It might also be an idea to get find out exactly what cam is in the engine -- either by identify marks or measuring durration, overlap and lift.

quote:

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Originally posted by JoelP
cheers all.

Its been bored out to 1640cc, and i thnk its got a fast road cam. Not a clue about jet size! Seeing as i cant check most of these things myself, i guess i'll have to leave it to the experts... ah well.

Cheers again!

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u2u sent cheers!

All in your mailbox now Joel

cheers mate!

well, i fiddled with the idle screw (thats as adventurous as i felt!), set it as directed. It started well, and i went out for a blat. Unfortunately, first junction it was idling at 3.5k! Which vaguely solved my problem i suppose, but it was a bit loud!

You really need to get it on a rolling road with an experienced tuner. You could spend a fortune on bits and pieces, and still not get it right, whereas they can set it to its optimum performance in very little time.
Dave Andrews will do it for you, if you can get it there!
Good Luck
John

[Edited on 4/8/05 by johnjulie]

[Edited on 4/8/05 by johnjulie]

i had a chat with mr boggs of brothers boggs fame. He says he has time spare next week to have a look, so as soon as i discover when i have a day off, it heading his way! The misfire is now worse, affecting it all the way up to the rev line. Still great fun though!