Re-Jetting Query

[moggyman64]

Hi All,

I have a 34 ADF carb on my 2ltr twin cam with timing set to 10 deg BTDC, Facet electric fuel pump and K&N filter. The jets fitted are as original spec 122 for primary main, 130 for secondary main, air corrector jets are 170 main and 175 secondary.

The engine is flat out at 80mph, so if I were to increase the main jets to say 130 or 135 and secondary to 135 or 140 would this make the engine run too rich so as to run like a bag of bolts or would it increase the power but reduce my mpg?

If I increase the main jets then would I also need to increase the air corrector jets too? Or do I only need to increase the secondary jets as that is the venturi that takes over after half throttle is achieved on the primary? If I were to leave the primary as is would that retain fuel consumption for daily driving up to half throttle?

All opinions greatly appreciated.


Regards

Keith

[baltobernie]

Guy Croft recommends the following for a stock 2 litre:

Primary:
main 122
air corrector 170
emulsion tube F20
idle jet 50

Secondary:
main 130
air corrector 180
emulsion tube F5
idle jet 90

... so you aren't too far off right now. Something else (exhaust restriction, ignition) going on? Can you achieve 5-6k in lower gears?

You didn't state what your idle sizes were. See the Redline web site for general tuning instructions for Weber carbs. The key is to get the low speed circuit ("idle") correct, then move up to the primary and secondary, as the low-speed circuit is active at all engine speeds.

Generally speaking, you should have about a 50-point spread between main and air jets.

What do your spark plugs look like?

[moggyman64]

Hi Baltobernie, thanks for the reply.

The engine is from a 1983 131. Fully rebuilt and balanced last year.

Wow I have never tried to get 5 - 6 k in low gears, afraid it would blow up. I will try it tomorrow though.

When you say " the low speed circuit " (idle) do you mean adjusting the idle mix screw at the bottom of the car? I have adjusted that and It is approx 2 turns out from fully closed position and the cars idles at 900 rpm without any problems. Is this what you mean about getting the low speed set correctly. If that was slightly out would that contribute towards the engine bottoming out at 80 mph? Is there something else I should be checking/adjusting?

I will have to check the idle jets also what plugs are in there plus their colour and report back

The exhaust header is the original standard cast type, into two downpipes and into one 2/14 " pipe. From the collector piece it runs in stainless pipe with two mufflers which is aftermarket.

Ignition is set at 10 deg BTDC, when you say ignition problem do you mean coil etc.? It has the original Marelli electronic ignition module and coil.

Regards

Keith

[So Cal Mark]

it's sort of crazy to consider rejetting your carb without knowing the a/f ratio before you start

[moggyman64]

Hi Mark,

I wouldnt just change the jets for the sake of it, I am just asking what peoples opinions are regarding re-jetting to try and understand why the engine is maxed out at 80 mph. I think I will fit a AFR gauge and see what the engine is doing.

Just had athought, as said I am using a Facet electric fuel pump if the pressure was excessive would that force too much fuel past the float thus raise the level of the chamber and emulsion tubes, which would make the engine run rich. Would running too rich actually flood the engine even around 80 mph and result is loss of power? I will check what the float level is, if necessary I will reset it to 6mm (original spec), take the car for a run and then check to see if the fuel pressure has altered the float level.

Regards

Keith

[So Cal Mark]

flooding would be much worse at lower rpms than flat out if you had excessive fuel pressure

[baltobernie]

There's an idle jet that controls the low-speed circuit, and if your engine is idling well at 2 turns out, you're close enough for our purposes.

It's probably not your fuel pump. If you installed a high pressure pump for an EFI application, the car wouldn't run, so I presume you've got the correct 2-3 psi pump for carb engines.

By "ignition", I'm referring to the spark breaking down under high load or high RPM. As you approach 80 mph, can you feel a misfire? Since you've stated that you are timed at 10 BTDC, I presume you have a timing light. Warm the car up, leave the engine running, transmission in Neutral and parking brake set. Attach the timing light, and see of the light "moves" as you increase RPMs. This is a test of your distributor advance mechanism. The spark must arrive earlier and earlier as the RPMs go up.

The big test for correct carb mixture by amateurs without diagnostic equipment is the color of the spark plugs. So pull one and tell us what you find. The wideband A/F meters are a terrific tuning tool, but you're not experimenting with a carburetor that was not originally fitted to this engine. So something else is going on, since you're pretty close on all specifications.

[moggyman64]

Hi All, I have checked the following item today and can report.

The car does not misfire at 80mph.
From a standing start I drove the car through all the gears at 5500 rpm and all seemed to be ok but leveled out at 80 again. Checked the speedo with sat nav and speedo read just over 80 and sat nav read 78 mph, so not too far out.

Checked timing with timing light and it is set bang on 10 deg BTDC and does advance with RPM.
I re-checked the idle mixture screw and it is 2 & 1/8 turns out from seated.
The spark plugs are FIAT 1L45J, I got them from a Fiat dealer, but I suppose that doesnt mean they are correct for the engine. The plugs are a mid brown colour.
The coil is Motaquip RPC-115 BN17 for electronic ignition.
primary idle jet is 50
Secondary idle jet is 90
Primary emulsion tube is F20
Secondary emulsion tube F5
Primary air corrector jet is 170
Secondary air corrector jet is 175

Am I correct in thinking that this standard engine is capable of more speed?

All comments welcome

Regards

Keith

[131]

primary idle jet is 50
Secondary idle jet is 90
Primary emulsion tube is F20
Secondary emulsion tube F5
Primary air corrector jet is 170
Secondary air corrector jet is 175

I've just gone through my factory 131 manual, the only difference it has from the above list is the secondary air corrector jet is 180. Not the answer to your problem.

[moggyman64]

Thanks Mick,

Yeah it is a slight difference but wont explain why engine is struggling like it is. Waiting to hear back from baltobernie or others to see if they can recommend what I can do next.

Regards

keith

[So Cal Mark]

I'd check the cam timing first

[baltobernie]

+1
... particularly with a rebuilt engine. I was 7-8 deg. out on one side. Made a big difference.

[moggyman64]

Thanks guys,

If the crank is set at TDC with #1 and #4 at TDC and the cam wheel are set to the marks on the cam housings, can the cam timing still be off?

I know that there are degrees for cam timing ie.
inlet opens at 15deg BTDC,
inlet closes at 55deg ABDC,
exhaust opens at 57deg BBDC
closes at 13deg ATDC.

I presume that these degrees are for completely open and closed valves.
If the crank and cam wheels are set as above then would the cam timing not take care of itself via the cam lobes? baltobernie you say that yours was out by 7-8 deg how was that and how did you sort it?. If the cam wheels are set correct then is the descrepency of degrees out because the crank is not exactly at TDC?
I have a crank degree wheel so if I set up the cam wheels and crank to TDC and fix the degree wheel to point at TDC and take readings of the open and closed valves degrees, is that all I can do to check cam timing?

I had a thought about the K&N air filter that is on the carb, because all the jets are as should be in a standard engine then could the engine be running lean at high RPM because the air intake is far greater than should be. I was thinking of fabricating an enclosed air filter housing similar to the original to see how that worked and also I find the induction noise from the carb annoying. Do you think this could be a contributing factor?

Lots of questions I know but your understanding and feed back will be appreciated, plus I am finding this very interesting and educational.

Regards

keith

[baltobernie]

The purpose of the cam alignment marks are to ensure correct timing belt replacement. They are probably very close to exact only on stock engines (those that have never had any block or head machining).

I sorted it with adjustable cam wheels. You have a degree wheel, so before you purchase adjustable wheels, you can check the specs you've listed against your engine. Are you running stock or aftermarket cams? Are your valves adjusted to spec?

The air filter is not your problem. I don't like the drone with my K&N either, but that's another story! If you are running lean on the top end, you would definitely hear and feel it; the sensation is unmistakable. Besides, your brown spark plugs are telling us you're in the right neighborhood, mixture-wise.

What were the results of your attempt to reach high RPMs in second or third gear?

[moggyman64]

Hi Baltobernie,

I ran at 5500rpm throught all the gears , had no backfiring but still bottomed out at 80.
The cams are stock, I havn't checked the valve clearence lately but I will need to check them now that It looks like the problem could lie there. I will check things over the next few days and report back
Regards

Keith