Idle issues

[AlanT]

Hi All,

Got my engine going really well but when dropping from above 2500rpm to idle it initially falls too low / almost stalls before recovering revs. I’m thinking that it’s over fueling and recovers as it burns off? 

[PeteH]

12 hours ago, AlanT said:

Hi All,

Got my engine going really well but when dropping from above 2500rpm to idle it initially falls too low / almost stalls before recovering revs. I’m thinking that it’s over fueling and recovers as it burns off? 

Fuel pump pressure?

Pete

[AlanT]

Pete,

how would I test this? Given the flooding issues (new floats and needles are in).

Thanks

[PeteH]

There have been reports of Pumps overpressurising on various threads.

Others have run pressure test`s recently, (Ian? or Colin?) try a search of the threads on here?. I suspect "Uncle Pete" will have some idea?. I supose you would need to find a low pressure gauge and fit in the outlet from the pump?. I think it should be around 2/3psi.?

Pete

[AlanT]

Thanks. It should be fine as is a secondhand pump that had been on another car, has the spacer, but does seem odd that 3x sets of needles are being over powered. 

[Ian Foster]

Alan

Have you checked the float levels.

Another possibility might be air leaks, but that would usually case idle to run too high.

Ian

[AlanT]

New floats and viton needles set up 

[Pete Lewis]

floating idle is more mixture related than pump pressures 

need a reminder.....do you still have waxstats on your jets ??? or even poppet valves on the throttle plates ???

 

edit  what oil  do you use in the dashpots ??? an over rapid drop and recovery will upset returning to idle

Pete

[Iain T]

I have a air/fuel ratio guage in my car and whenever after deaccelarating it's nearly stalled at idle the reading has always indicated its too lean (17+). After many hours tweeking with different needles I've found my engines sweet spot is to set the idle at 700rpm(ish) and adjust the mixture and synchronisation to suit. When I have set the idle at higher rpm (800/850) the mixture goes out of plonk up the rev range. I've found I had to richen the mixture for the standard idle rpm which gives a better mixture over the rev range and WOT. Easy for me to see as I have a afr worry/information gauge. 

Another thought, do you have a return spring on your throttle? 

Iain 

PS, I checked the fuel pressure of my Huco electric pump and it was 2/2.2 and it doesn't overflow🤞

[Pete Lewis]

i believe you will find all needles start with the same Idle diameter on the needle which is why setting the mixture up at a specified Idle is important 

too fast or too slow idle setting will affect what happens in its as running   position

Pete

 

[AlanT]

Thanks all,

new red jets fitted. Going to go back to the start and retune the carbs again.

[AlanT]

I do have a return spring. Standard engine oil in the dash pots(was fine on my previous SU carb cars)

[AlanT]

Hi Mathew.

replaced with new red jets. 

[Iain T]

1 hour ago, Pete Lewis said:

i believe you will find all needles start with the same Idle diameter on the needle which is why setting the mixture up at a specified Idle is important 

too fast or too slow idle setting will affect what happens in its as running   position

100% correct! 

[Badwolf]

17 minutes ago, Mathew said:

Just got rid of the wax capsules with 1p's and the problem was solved!

With inflation running so high, we might have to rethink the 1p fix....... I'll get me coat on the way out!!!

[chrishawley]

Just to clarify: That would be a Spitfire 1500? With unmoded HS4s? On conventional points or electronic?

Can the issue be reproduced with the vehicle static, or is it only evident in the driving condition?

Tricky problem so I'll hold back on any opinion until I'm sure which vehicle I'd be opining about. But, only vaugely relevant, I did have a non-Triumph with the 'dying-on-overrun' problem which after much investiigation was tracked down to points scatter. Distributor and distributor drive had advanced wear and on a strobe the timing was dancing around all over the place. The scatter was evident at all rpm but most pronounced on decelaration of the engine.

 

 

 

 

[AlanT]

Good news! Fiddled about again & have it idling at 1000rpm consistently. A little fast but the problem appears to have been that the engine is quite tight post new bearings etc. 
I ran it a bit faster up the road (3000rpm) and it’s now settling down. Engine is rattle free, smooth.

Of course a few more post resto niggles emerged whilst testing…Speedo has stopped (expect cable undone) and o/d stopped engaging (probably the switch I tried to rebuild as the new one was terrible quality).

And I’ve omitted to install sun visors! 

Not sure about the twin box exhaust experience yet…great with hood down but v loud with hood up. May end up going stock.

Thanks all.

[daverclasper]

5 hours ago, Pete Lewis said:

i believe you will find all needles start with the same Idle diameter on the needle which is why setting the mixture up at a specified Idle is important 

 

Would appreciate more explaining please Pete, if all needles for a particular carb are the same diameter at idle and mixture set at idle , then, how does this allow for the different needle profiles through out the rev range?. Some basics, I'm not understanding.

cheers

[Iain T]

Needles for a particular jet orifice start at pretty much the same diameter at the top. Say a 0.090" jet has a 0.088" needle hence restricting fuel flow. As the needle rises the various tapers of needles allow more fuel to be sucked up. My needles are 0.0519" diameter at the bottom letting more fuel through at max revs. The different needles have varying tapers (in fact some go in and out) over the length. Tapers are measured at 13 points along the length so you can get an idea of what you need. Hence the need to get the correct needle taper profile for your engine, especially if you've a modified engine. Throw damper oils and springs into the ring and it becomes a minefield! 

 

Iain 

[Peter Truman]

Here's a similar graphical representation of Iain's table of the different needles profile variation that I did when converting a Spit 1500 from a single 150CD Strom carb to twins, and I only had twin SU carb needle references.

[daverclasper]

Thanks, I understood how the needles/jets work, though your info clarified, that as long as the needle profile is matched correctly to engine size/characteristics, then to set at idle.