740 SE B230K very high idle

[xtDANIELODONNELLx]

My 1988 740 SE is currently idling at anywhere from 1500 - 2200 rpm, Only when hot, it's about 1000 rpm on cold start until the engine heats up. I was wondering if anyone could help me troubleshoot what might be happening.

I had to overtake a car recently, hit the gas to get past and over to my side of the road again as quickly as possible, I felt a massive drop in power when I started accelerating and the high idle issue started later that day after a 2 hour drive.

I have also recently discovered that the last owner had the vacuum system all wrong, I have now corrected this. The problem still persists.

The car starts straight away without a problem, sits at about 1000rpm, then when the temperature starts to creep up the idle speed goes with it. The idle is not sporadic, with no movement of the needle on the rev gauge. it's just way too high.

The car is fitted with a Pierberg 2B7 carb.

Is there something I can troubleshoot and try myself? Should I find someone to rebuild the carb? Should I bite the bullet and do a weber conversion? Any advice or help is very much appreciated

[classicswede]

It does sound like a carb problem.

Check that the throttle flaps are both closing. I suspect the secondary is not fully closed

[Laird Scooby]

Originally Posted by xtDANIELODONNELLx

The car is fitted with a Pierberg 2B7 carb.

Is there something I can troubleshoot and try myself? Should I find someone to rebuild the carb? Should I bite the bullet and do a weber conversion? Any advice or help is very much appreciated

It sounds suspiciously like either the auto-choke has failed and it's over-fueling or the carb is worn to a point where it's over-fueling anyway. If you can accelerate gently throughout the rev range but as soon as you try being even slightly "enthusiastic" it dies, that points to a worn carb.

I'd strongly suggest the Weber conversion, by the time you've faffed about trying to rescue a lost cause aka the Pierbug (no, not a typo, that's their nickname because they're full of problems) and spent time and money on it, you'd be at least halfway towards the cost of a new Weber conversion.

https://www.webcon.co.uk/products/83...and-pulse-air/

That's the one for cars without EGR which i suspect your car will be, there is an alternative kit for cars with EGR for the same price.

[xtDANIELODONNELLx]

Amazing Dave, thank you so much.

I've suspected that a weber conversion was the way to go for a while but I think I needed to hear it from someone who knows better than myself.

I thought mine was without EGR too until I seen something that looks a lot like an EGR valve underneath the back of the carb, but I could be mistaken. It doesn't have aircon, nor does it have a catalytic converter if that confirms it?

[Laird Scooby]

Originally Posted by xtDANIELODONNELLx

Amazing Dave, thank you so much.

I've suspected that a weber conversion was the way to go for a while but I think I needed to hear it from someone who knows better than myself.

I thought mine was without EGR too until I seen something that looks a lot like an EGR valve underneath the back of the carb, but I could be mistaken. It doesn't have aircon, nor does it have a catalytic converter if that confirms it?

It's too early for a cat which is useful, not so sure the EGR valve will be on the carb unless it's being fed with a hose from the exhaust manifold - i don't mean the 40-50mm diameter corrugated carboard covered in aluminium hose either that directs hot air to the air intake via the air cleaner. A pic of what you believe may be the EGR would help, could simply be a throttle damper.

[classicswede]

The easy way to see if you have EGR is to look at the casting number on teh head on teh exhaust side. 631 head will be EGR and a 530 will be non EGR. To 100% confirm look below the exhaust manifold and EGR cars have a small pipe coming out of each post below the manifold.

Dave is 100% right that a weber conversion is the way to go.

[xtDANIELODONNELLx]

Attached an image here to show what I think is the EGR valve, thanks classicswede, good to get another opinion as reassurance that it's the right way to go, I'll look for those points when I get home from work

[Laird Scooby]

Originally Posted by xtDANIELODONNELLx

Attached an image here to show what I think is the EGR valve, thanks classicswede, good to get another opinion as reassurance that it's the right way to go, I'll look for those points when I get home from work

Sadly you're right, you do have EGR and that is the EGR valve. It might be worth cleaning it but i doubt it'll improve matters with the carburation. I've photographed the relevant section in the Haynes Book of Fantasy (HBoF) which might be helpful for you. I'd still be looking at a Weber conversion though for reliability and economy.



You should be able to zoom in on the relevant bits of that but usually EGR valves give trouble increasingly rather than going full-on into problems.

[Forrest]

My own experience of the Weber versus Pierburg debate relates to Volkswagens rather than Volvos so may not be directly applicable. However, I found a properly operating Pierburg generally superior to the Weber alternative.

My Mk2 Scirocco came with a Weber fitted by a previous owner. It ran lean to the point of rapidly overheating when idling. It also tended to conk out under load; a situation that could be improved, but never fully eliminated, by various tweaks to the fuel return line.

My 2020 lockdown project was to build a working Pierburg from three donor carbs. I did this from a complete strip-down, ultrasonic clean and rebuild with new gaskets following all the adjustments in the original Pierburg service manual and leaving well-alone the things it said were factory set.

Not only does the result run much better than the Weber ever did, by some miracle it passed the MOT emissions test first time plus it offers the benefit of a fully working automatic choke.

I had never rebuilt a carb prior to this and it did take a while to get my head around how the Pierburg is supposed to work. However, it's very rewarding when you do. Added benefit is no deviation from factory spec which might need to be declared for insurance purposes.

[Laird Scooby]

Originally Posted by Forrest

My own experience of the Weber versus Pierburg debate relates to Volkswagens rather than Volvos so may not be directly applicable. However, I found a properly operating Pierburg generally superior to the Weber alternative.

My Mk2 Scirocco came with a Weber fitted by a previous owner. It ran lean to the point of rapidly overheating when idling. It also tended to conk out under load; a situation that could be improved, but never fully eliminated, by various tweaks to the fuel return line.

My 2020 lockdown project was to build a working Pierburg from three donor carbs. I did this from a complete strip-down, ultrasonic clean and rebuild with new gaskets following all the adjustments in the original Pierburg service manual and leaving well-alone the things it said were factory set.

Not only does the result run much better than the Weber ever did, by some miracle it passed the MOT emissions test first time plus it offers the benefit of a fully working automatic choke.

I had never rebuilt a carb prior to this and it did take a while to get my head around how the Pierburg is supposed to work. However, it's very rewarding when you do. Added benefit is no deviation from factory spec which might need to be declared for insurance purposes.

I would have been stripping that Weber down to correct the float level and ensure it was jetted correctly.

The Weber conversions are designed as a direct replacement to factory specs so doesn't necessarily need to be declared. Maybe a minor tweak on the idle mixture screw might be needed for individual cars to set the CO as it should be but other than that, they're good to go.

Out of curiosity, what's your view on Solex carbs?