idle jet size - DHLA

[jono]

This is related to the other thread running but it's quite specific so I thought I would open another thread.

My engine is a mildy tweaked TC - mildy ported head, sprint valves, larger exhaust valves, large bore headers and free flow exhaust system, big airbox, QED360 cams. Putting out around 135bhp. It runs on wasted spark and a mapped ignition system. Plus 2.

The cams are DHLA40 - early ones with 3 progression holes and no air bleed facility. It goes well, especially on the mains but I've always had isses on the idle circuit.

I've been playing around and set the float heights precisley to 14.5mm - some improvements. However it has a marked stumble from around 2,700 through to 3,150rpm which is very bothersome as that's excatly where I find myself a lot of the time, coming out of a corner for example.

I'm guessing that this relates to the progression phase fronm the idle circuit to the main jets?

My idle jets are 55 and the mains are 135. The stumbles make me think that going up a size on the idles is the right thing to do as a start point however 55's seem to be on the large side are they not?

Could the sumble be caused by the mixture going over rich in the progression phase as opposed to lean?

Any suggestions - I know this is going to be a trial and error but should I go for a larger or smaller idle jet?

Cheers

Jon

[bill308]

Hi Jon,

I'm not a DHLA expert but I was told by one that the early DHLA's (no suffix, 3-progression holes) were designed to get "on" the mains as soon as possible. From this, I conclude that in the region of interest, 2700-3150 rpm, you are on the mains, not the progression circuit.

You mentioned that you changed the float heights and noted some improvement at the new 14.5 mm height. What were they before? This should give you an idea of the direction to go in. What does the book specify for float heights with your floats?

It could be that you are lean at the low end of the rpm's range which could be corrected with the next richer main jet. This change could make the top end of the main's range too rich, which could be compensated with an air corrector change.

Lacking feedback from an A/F meter, you pretty much have to make an change and see what happens.

Raising the float heights:

1. raises the fuel level in the bowl
2. makes the idle mixture richer
3. makes the transition mixture richer
4. makes the main mixture richer
5. extends the transition range to a higher rpm
6. brings the mains in earlier

Changes may be small but should be there.

Bill

[jono]

Many thanks for these comments Bill, much appreciated.

You raise some interesting thoughts which had not occurred to me. I have various jets and air correctors and will have a play around and see where it gets me to.

I've been on the cusp of going down the fuel injection route for some time now (being part way there with wasted spark ignition) but the cost is high so I just want to give the carbs one last chance!

I will update this thread with the outcome of my tinkerings. I've just purchased my own Morgan Carbtune so that's a good start!

Cheers

Jon