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Power Band Understanding


Davo

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Im fair new to all this and have been working through issues with a car that I acquired. So far Ive now got it through its mot.

 

Im now onto performance issues.

 

I have a b reg 84 Sierra gl as the donor in a Robin Hood mk 1 or 2.

 

It has a 2.0 pinto engine with a Weber dgav 32 36 progressive carb I believe.

 

So it starts and runs ok. Up to about 3000 rpm all is good but just over that it dips power which worries me so I change up gears.

 

However, stationary not under load there is no problem revving beyond 3000.

 

Its all pretty dirty at the moment and hasnt really been run for a few years now. Ive used a load of carb cleaner and loaded the tank up with redex which Im hoping will clean the jets through.

 

I probably need to change the distributor fairly soon as my father who gave the car to me tells me the the capacitor isnt currently being used.

 

I know I have that to address but I was wondering if anyone had any advice on other things to look at.

 

 

David.

 

(BOURNEMOUTH )

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Hi David,

Couple of things to check first.

Is the engine running smoothly or is it misfiring?

Any black smoke at the 3000 revs?

The DGAV carb is a good one, check that both chokes are actually opening fully.

Take off the air filter & peer down the throats while you operate the throttle, engine off!.

Make sure the choke is operating correctly, it should close the choke flap when the engine is cold

&open up when it gets hot.

Check ignition timing with a strobe, set the idle at approx. 10 degrees btdc

Then rev the engine & make sure the ignition advances.....max 37 BTDC.

POst back when you have checked

HTH Bob

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Guest lotusPaul

Apsolutley..one thing at a time.

Give away signs, no problem from idle but underload. Generally that points to fuel filter as underload demand is more.

Same time is a good clean of all fuel lines, or replacement.

If its been stood for a few years then it could have old deposits in the carb jets.

 

To be fair, if you have the wonger, get a replacement carb and re do the fuel system.

Ignition, forget the old dizzy and get an Aldon electronic replacement too with matching leads.

 

Yes its money, but well spent.

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Thanks for the tips. I can answer some and will progress with the rest during the week.

 

It feels pretty smooth to the novice ear.

 

No black smoke at 3000 when stationary. Ill have to check with another person when under load.

 

There are some issue with the choke I know. Although I understand from the carb model its actually a water a choke I believe its been converted to manual. However I can see the cable isnt connected. I can however move the lever manually. I see I little movement from the choke flaps but they dont return to their open position naturally so I have accepted the choke isnt working and generally start the engine in the garage where it is a bit warmer and have manually moved the flaps to the open most position. I know this isnt ideal but I was just working on the assumption of no choke function. Maybe this has further implications?

 

Timing is something Ill have to do later in the week with my dad who has the strobe.

 

I am suspicious about the vacuum advance. I really am a novice but I understand that when the carb is under load a vacuum is created in a pip between the car and the distributor to somehow cause it to run a bit quicker. I did hold the pipe whilst revving the engine but to be honest I didnt feel a vacuum feeling in the pipe/tubing. I have taken the tube off and can pass air through it.

 

As I happened to have a spare set, I have just changed the spark plugs. Some a little wet.

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Guest lotusPaul

Just for knowledge. Greater vacuum is felt at lower revs. Shame im not in Dorset anymore or id help, was in Bovington.

Does sound like you would benefit from a good setup and tune. Theres a guy near the railway station thats old skool stuff. Dont remember name tho.

 

Theres a good few guides on web for stripping and rebuilding both carbs and dizzy. Have a look and see if its in your abilities to carry out.

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Thanks lp. Just noticed your extra post.

 

As it happens I have replaced the flexi fuel line from the metal part not so long ago.

 

The fuel filter probably could do with replacing. Interestingly if I leave the car for a week or so the fuel line does seem to drain and it takes some good syphoning to get it flowing again.

 

Changing to an electronic igition was actually on my plan of things to do hoping its a suitable job for a novice. I see theres a lot of choice out there though; would you mind posting a link to a suitable one and I can add it to my Christmas list.

Ta

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Something else you need to check which could be your problem, as i've had similar...

These carbs have 2 venturi. The second one opens only when there is need and is controlled by a diaphram responding to vacuum (or i think the older one just opens at a certain point).

My guess is that when you hit around 3k the second venturi opens but the jet is blocked so it is only adding extra air and no extra fuel. So you are diluting the mix when you need it most.

 

Take each jet out and check they are clear. Remember which order you take them out (one at a time is safer so you know the right one is put back). The jets pull out of the emulsion tubes so you can hold up to light and see it is nice and clear.

 

hth

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Agent, interesting you say that. I forgot to mention this. Its a progressive carb I believe and the second chamber opens via a throttle linkage. When the first throttle is three quarters open, I can see the second throttle starts to open. However, until a couple of days ago the second chamber was seized shut and the linkages couldnt open it. After a lot of wd 40 and wiggling its now free. I get your point that the jet may be blocked and cant supply enough fuel, however as it works via throttle position rather than diaphragm 3000 shouldnt be the trigger; it would be the throttle position right? The throttle position doesnt seem to affect it.

 

A lot does seem to be pointing toward me at least getting the jets out to clean them. I think I have to do this from within the float chamber which does mean dismantling I guess.

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A performance upgrade is to fit a pair of twin carbs....the 40s refer to the max choke (throttle choke diameter) .

There are larger ones at 45, then more rarely 48s and 50s..They need more setting up and maintenance than a single carb.

For the time being stick with the DGAV they are a very good carb, but it obviously needs a clean up.

A new one is an option, buut may be difficult and expensive to find.

The fuel pump is a mechanical one, and should prevent any fuel run back. It may be faulty or need a refurb,

which could be thje reason for the problem.

Just go at it logically and methodically.

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Investigate the obvious known problem. Agent_Zed makes a very good point. Failure of the second choke will produce symptoms just like you describe. Your observation confirms problems with the second choke not opening correctly if at all. Don't rush to spend money. Take the carb off the car. Take it indoors in the warm and carefully strip it down, making notes and taking pics if you need to. You can get any/all the bits from a couple of suppliers if you need them. The soft metal of the carb does not rust but forms aluminium salts, white gritty powder which cleans off easily but is high friction, so stops things moving. Look at all the jets and blow through them. Don't poke them with anything metal but you can use a nylon bristle from a broom, gently. Put it all back together and your car may run OK.

 

Once you have done this and tested how the car runs you can look for other problems if you need to. Only if the carb is unrepairable should you buy a replacement. The DGAV is a perfectly good carb.

You will need to set the timing. Sierras didn't come with a capacitor in the distributor. There might be one in the wires close to the distributor but it's not essential.

 

Nigel

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