Quirks of Mikuni/Solex's and Webers

[jeffro ra28]

Thanks Bill Rodger and all others for their input. This is a really good read

Just wondering if someone could asses this situation for me???

So its a ta23 with a 2tg. Cams are 233 degress @ .050"(not small), 4-1 extractors. Carbies are 40mm solex's which are fed by a inline electric fuel pump suited for a 6 - 8 cylinder engine.
Now where it all started was, after giving it a hard run from 1st threw to 3rd gear i found i was running out of fuel in the bowls when it reached about 6000 rpm in 3rd gear(this was tested many times). So after replacing the fuel pump with a new one(suited to 6 - 8 cylinder carby engines) i found a fuel filter blocked (before the pump). After replacing this filter and having a new pump everything seemed ok for a couple of days. However still stuterd when the throttle was suddenly pressed then cleared itself up and wooppeee tyre smoke.

So at some stage after all this i am driving along and it decides not to rev past 6000rpm. And when i apply more throttle it just gets worse. Its like a nasty misfire, however no backfiring. At a guess i would say it is flooding as i get a deep droning sound from the ram tubes as the throttle blades open
I have checked the filters, there are not blocked and i beleive its a rich condition rather then lean.

I have gone through the ignition system and i am confident this isnt the cause.

Any one with any suggestions? Perhaps the fuel pump is to big and pushing fuel past the needle and seat?

[river]

Hi,

Yes, I was wondering about the fuel pump. You don't need a high pressure pump for carbs and it may be pushing too much petrol into them, especially if you got one for 6-8 cyl engines.

Is it possible to put in a temporary restrictor of some sorts to see if it makes any diff?

seeyuzz
river

[jeffro ra28]

i probly have 4-5 mechanical fuel pumps i could try. Its just the fact it decided to happen a few days after i had installed the new pump etc.

[mike1980ra40]

Thanks Bill Rodger and all others for their input. This is a really good read

Just wondering if someone could asses this situation for me???

Sounds very much like one problem i had in the past except i never got to that
hi an RPM. I found my facet pump was to high a pressure and changed to a carter
inline pump with a lower pressure, 2.5-3.5 psi (comes with its own pre pump filter).
Is a lot quieter, smoother and more responsive. if the pressure is too high you can
split the diaphram thats on the bottom of the carb (forgot what that bit is called).

is your pump mounted by the tank or in engine bay?

cheers.

[jeffro ra28]

Thanks Mike,

The pump is mounted bellow the tank.
I spose i should try a mechanical pump see how i go.

[boyracer]

My understanding is that Webers only like about 5psi max at the needle & seat other wise it pushes the fuel past and makes it run as fat as, but usually thats at idle.

I'd say leave the pump and throw on a presure regulator.

The other problem i've have with my car at sustained hi rpm is that the needle and seat is to small and doesn't flow enough fuel , hence it runs the float bowl dry.

These are just sum of the things i've experienced... but hay i'm no expert

[Rodger]

jeffro,

Remove the jet covers and check if the jet block assemblies have come loose.

Regards

Rodger

[jeffro ra28]

Rodger. After all the ginning around iv been doing. I removed the bottom diaphram to get to the tapered locking screw and tightened it. That fixes the problem! so simple.
Thanks alot.

[Rodger]

Well that would do it jeffro. The whole Jet Block Housing shaking around. Kind of turns fuel into a bubble mixture.

Another "Quirk of the Solex", posted.

I plan to do a lot of collating of all these tips to add to my own and publish them on my web site as an alternate source of info. I'll try to pay due homage to those who offer them, so if I miss out then thanks in advance.

Regards

Rodger

[Billzilla]

Yes, I was wondering about the fuel pump. You don't need a high pressure pump for carbs and it may be pushing too much petrol into them, especially if you got one for 6-8 cyl engines.

With DCOE Webers, never run more than about 2.5psi. We ran 1.5psi on the race car and it worked fine.

[jeffro ra28]

Ok so i bought the new pump suited for 4 cylinder engines, but i was planning on returning it as i fixed the problem with the jet block assembly anyway. So do you think it would be a wise move to still install the smaller pump instead of using the big one?

[Rodger]

I see this as being about volume/flow rate and whether it can keep the fuel bowls full under long hard runs. A higher pressure rated pump pushing through standard fuel lines can compensate by moving the fuel faster.

My 2T with the dual 44mm Mikunis ran with the OEM mechanical pump but -6 lines from the tank, quite fine. I suspect though that on the straight at Wakefield I was close on the fuel supply limit and hence running towards lean. On the street its very ok.

Solexes can handle 6-8psi. They work well also on lower pressures if the volume/flow is good. If you replace the fuel lines with bigger -6 or -8 then a lower pressure/higher volume pump can be run.

If drag racing or circuit use is your thing, then try both pumps on a long fast run and see if the power drops away or it keeps pulling. Or after the runs, stop and pull a plug to see if there is any difference in plug colour.

On the street the smaller pump is the prefered.

Regards

Rodger

[GeeB]

I just want to say thanks for the advice and info offered on the forum. After years of spectating i have bought an RA23 rally car, my limited mechanical knowledge and funds mean i am dependant on those with the knowledge. With a new set of plugs the car ran quite well for a few weeks it just started playing up on the way to my first motorkhana, by lunchtime it would hardly run at all. Seemed to be running rich, idle was terrible etc, only just made it home. After some interesting and varied suggestions thought it safer look on here. Took the covers off solex's, one had jet holder sat in position but completely unscrewed the other had about 2 turns left. Is this a common prob? Is there a fix or is it just a case of regular checks? Anyway before i put the foam "socks" back on, does anyone know the right way to clean and re-oil them

[Rodger]

GeeB,

Clean them with ordinary detergent and/or Diesel fuel and re-oil with the filter oil. Should be able to get the stuff at any MC shop or Filter place that sells washable filters. Such as The Filter Factory.

The Jet block housings coming loose like this is a very common problem. Most often it is caused by not installing the parts back in correctly if they have ever been pulled out in the past. They get screwed back in by turning on the Air Jet instead of the Jet Block.

Use the widest Flat Blade Screw Driver that wont foul the body and unscrew and remove them all. If the Air Jet undoes first, no problem as this is what you need to do once the whole jet block is out anyway, then undo the jet block.

With a 10mm open end spanner unscrew the Air Jet from the jet block.

Take the oportunity to write down the sizes of Main Jets and the Air Jets and check the assembly for cleanliness. Dont forget to check the internal/removeable Bleed Pipe.

Re-assemble in the following order:

Screw in the jet block and tighten down fairly firmly.

Now screw on the air jet so it is firm. The aim is to be able to remove the Air Jet without undoing the Jet Block.

As part of a regular pre-event routine add checks of the Jet Block Assemblies and also checks of the Inner Venturis. As mentioned in this thread loose Venturis are another common problem.

Regards

[GeeB]

Thanks again, followed sequence, (someone has obviously had a go in the past with the wrong size screwdriver), a couple of bleed tubes wouldn't come out so didn't force them, left them in for now. Venturis seem ok. All back together and car running pretty good, not perfect but not sure how much to expect with old car and old carbs. So now i can get back to learning how to drive sideways on some loose stuff, cheers