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MG Midget and Sprite Technical - Fuel starvation (and I've checked BB archive)
|Hi, having spent an hour in the archives, I'm still stuck. 1275, 1330cc, LCB Sprite on 2 x 1.25 SU's, began to lose power at 4,000 revs. Just wouldn't pull past that point. Car had stood for 3 months after a long event (2,000 miles). On rolling road found that old solid state pump was delivering little fuel at high revs, so new Facet silver top placed in boot, and filter king in engine bay, but still wouldn't run right. However, if ran a hose from pump directly into fuel tank via filler neck then OK. Took it home and fitted a replacement second hand tank and new hoses. No fuel pressure at all. Put a long inlet hose on the pump and it ran! However, still has apparent fuel starvation at high rev's (even if running off a petrol can straight to the pump in the boot), so tank and tube i tank seem not to be at fault. Engine will pull to 6,5000 if I press the throttle slowly, but when I floor it, it sticks at 4,500. Tank is vented via a port on the filler neck ad long 'U' hose, which I put in.|
The carb throttle shafts are worn (approx. 2mm play) and carbs have tended to over-run and for the idle speed to climb over a long day of competition (I do historic road rallies). I'm confused. Do I just need a new set of carbs?
Sorry for long post.
|Glad you survived the Archives, theres people still lost in there we havent seen in years...LOL|
2 MM, now thats some play in my opinion, id certianly get a rebuild kit, and do a good through cleaning ... most likely you wont need to ream out the carb body as the brass shaft is what mostly gets worn, not the carb body, Id make certian the fuel pump dosnt pump over 2 psi, 1.5 psi is idel, also it needs to be lower then the bottom of the fuel tank, check to make sure the fuel float bowls are sitting level and not ascew.
if you no longer confused... just send me the extra cash you where going to spend. LOL
Prop... Heatshield? I thought about mentioning it but then thought ... NAH!!!
|Sorry I meant to ask... whats your valve clearace set to?|
| most likely you wont need to ream out the carb body as the brass shaft is what mostly gets worn|
sorry, what was I thinking?... I meant to say you wont have to replace the bushings for the butterfly shaft..
|The AUF 214 Su Fuel pump (OEM for midget) puts out 2.7 psi. 1.5 psi for a rear mounted pump is a bit on the low side. Cheers - Dave|
Agree some confusing events in your post; maybe shows you've had more than one problem?
Anyway - if you floor throttle, there should be enough fuel in the bowls to allow the engine to instantaneously rev up to max. Next time engine is running OK, switch off and check fuel level in both bowls. Also, check you have air-filters on the right way round (exposing the vents on the carb inlet, near the main inlet).
Worn throttle shafts should affect idle (high vac at idle encourages air to leak in and weakens mixture; as the engine is loaded, less vac means the carb runs with corresponding less leakage) but not WOT.
|So can you maintain 4500 or more for a few seconds (till the float chambers empty) but it then misfires unless you back off to 4000. Or does it just get to a certain speed and not go any faster?|
|Hi all. From a fresh idle it is OK first time through the rev range, but complains second time, sticking at 4,000 ish. If I back off, it gathers itself and then will slowly climb to higher revs. I have the filter king set at about 3psi, so should be plenty. I have been away, so havn't run any suggested tests yet.|
I have long inlet trumpets and ITG socks over them. Sounds like the pump isn't delivering enough fuel, but it's brand new, I know, anything can be faulty.
|have you tried removing the filter king thing?|
|Oh I missed that idea... the vent hole being blocked, good call!!! |
its the little vent hole behind the air filter (housing) its at 2 oclock on the rear and 10 oclock on the front...vary common mistake, its what vents the air from the carb piston
|this vent hole. I don't have an air filter housing, just trumpets and socks. So where is it then? On the carb body somewhere?|
|SH - the vent hole is on the front face of the carb; smaller dia than your little finger. You've probably not noticed it before; there should be corresponding hole in the air filter case / trumpet flange / anything that bolts to the carb.|
|What happens if in the event you pull the choke?|
I had that on my 1500 turned out i didnt had the right needles in my carbs...
|Arie de Best|
|The vent is on the front face of the flange that the air filter would bolt up to. Its possible that your trumpets are blocking it.|
|this is for an HS4 carb, vary similar to the HS2 ... it the photo you can see that this HS4 has 2 vent holes the red pen shows the location of where the vent would be located on an HS2 for the rear carb.|
the front carb would have the vent hole on the oppisite side of the carb throat (venturi)
|In this photo, you can see how esy it would be to accindtly cover the vent hole with a filter housing, a gasket, or a trumpet...its a vary common mistake, that everyone has to learn.|
Its upsetting they wont teach this in kindergarten...LOL
|sorry a little clarity iss in order...|
the HS2 carbs (the ones you have) only have 1 vent hole per carb, while the HS4 carbs have 2 vent holes per carb
|great, especially the photos. I'll go and check tomorrow as car is kept away from home, so I have to wait. I'll let you all know how it goes.|
|So, not the little vent hole. But, took everything apart, cleaned all and blew through, re-sited fuel pump in boot (rear inner wheel arch). Fiddles with the filter king a lot and now running perfectly. Don't quite understand what has made the difference, but seems very sensitive to setting of filter king. Thanks for all help. Think I'll still look for a set of 1 1/3 mgb carbs to replace my old 1 1/4 ones with.|
|see my earlier sugggestion:|
'have you tried removing the filter king thing?'
|what is a filter kign?|
Btw.. congrats SH, I hope that works out well for you!
This thread was discussed between 12/05/2010 and 17/05/2010
This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.