Playing about with inlet manifold

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Playing about with inlet manifold

Postby Asolepius » Sat Jan 02, 2016 11:51 am

Some more of my endless tinkering. When I put the steel bonnet back on I found that the top of the carb touched it. Strangely this didn't happen with the plastic bonnet. Only a light touch, but not good in the long term. To recap, I have an HIF6 carb on a reworked MG Metro manifold. I needed to drop the manifold from its existing 30º, which could not be done without making it longer. In fact, to clear the exhaust I had to make extensions that are 70 mm long. These are brazed up in aluminium (very difficult job!). The actual reduction in angle is 7º. Meanwhile I saw a long lecture by David Vizard on YouTube, in which he said that there is no real limit to how long you can make the manifold, the power just keeps on increasing. So I thought this was worth doing. Needless to say I matched up all the ports very carefully. I had to lengthen the outers of the throttle and choke cables, but the inners were long enough. They seem to work OK.

Anyway, the adapted manifold is on the car now and the effects are interesting. Firstly the idle speed is much higher and I can't back off the adjuster to get it below 1000 rpm. The mixture was way out and I fancy it is richer after much tuning. It is correct now according to a plug reading. But the oddest thing is that the engine speed falls on a closed throttle more slowly. The carb came without a piston spring and perhaps it should have one - if so which? I know springs were not always fitted, but could this be related to the slow fall-off of revs under closed throttle?

No doubt someone will say that it would have been much easier to buy an aftermarket manifold. But these are nearly all for transverse engines and still 30º inclined, which if longer would make the clearance problem worse. I was concerned that I would have to try several before finding the right one. And these days I have the time......
Les Rose, Salisbury, UK
1959 Sprite, owned since 1978
Asolepius
1098cc
 
Posts: 489
Joined: Tue Jan 25, 2005 6:50 am
Location: Salisbury, UK

Re: Playing about with inlet manifold

Postby Charlie Tolman » Sun Jan 03, 2016 2:23 am

Hi Les,

After modifying the intake manifold, reducing its angle, are the float bowls level, or are they tilted somewhat ?

Idle speed is high and cannot be reduced below 1000 rpm? Engine speed reduces slowly after the throttle is closed ? Fuel / air mixture has changed ?
A possible air leak in the new intake system or gaskets ?

Springs in the carburetor dash pots are not really there to push down on the piston. They are there to control the dash pot piston's speed in moving upward when the throttle is opened abruptly. Install some weak springs, and see what happens. I ran my 948 race engine without any dash pot springs (for quick throttle response).

Do the early transverse engines (Mini) have a tilt angle of the intake manifold that is different than that on a longitudinal engine (Sprite) ? MG Metro angle ?

Charlie T.
Charlie Tolman
Judson Supercharged
 
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Location: Minnesota

Re: Playing about with inlet manifold

Postby Asolepius » Sun Jan 03, 2016 11:15 am

Yes Charlie after some thought I am now looking for air leaks. There is only one carb and the float chamber is integral so can't be adjusted. I don't think 7º should make a significant difference. There will be far more change of G on cornering.
Les Rose, Salisbury, UK
1959 Sprite, owned since 1978
Asolepius
1098cc
 
Posts: 489
Joined: Tue Jan 25, 2005 6:50 am
Location: Salisbury, UK

Re: Playing about with inlet manifold

Postby Asolepius » Wed Jan 13, 2016 5:54 am

After many hours of further work I am now sure there are no air leaks. The main problem was that the new flanges at the head face were slightly thinner than the exhaust ones so were not clamping properly. I had to make spacers. The engine runs better now but there is still a slight reluctance to drop back to idle. I haven't tried a piston spring yet, and maybe I'll live with how it is as I don't want a slower throttle response.
Les Rose, Salisbury, UK
1959 Sprite, owned since 1978
Asolepius
1098cc
 
Posts: 489
Joined: Tue Jan 25, 2005 6:50 am
Location: Salisbury, UK

Re: Playing about with inlet manifold

Postby Charlie Tolman » Thu Jan 14, 2016 1:10 am

There are many Sprites and Midgets on the road and on the track, with
"half-washers" or "half-round" spacers, to even-out the adjacent flange thicknesses on the intake and exhaust manifolds.

CT
Charlie Tolman
Judson Supercharged
 
Posts: 1198
Joined: Wed Mar 01, 2006 10:36 am
Location: Minnesota

Re: Playing about with inlet manifold

Postby Asolepius » Fri Jan 29, 2016 12:57 pm

Yes Charlie, I'm sure the manifold spacing issue is very common. I have it all working fine now with no air leaks. I found the throttle problem was a sticky cable. As I needed a longer cable I used a bicycle brake cable inner. The fitting at the carb end was too tight and the old inner was kinked where it previously was clamped onto the throttle lever. I thought I could get away with the old inner in a longer outer, but not so! The carb is clear of the bonnet now.
Les Rose, Salisbury, UK
1959 Sprite, owned since 1978
Asolepius
1098cc
 
Posts: 489
Joined: Tue Jan 25, 2005 6:50 am
Location: Salisbury, UK

Re: Playing about with inlet manifold

Postby Charlie Tolman » Sat Jan 30, 2016 1:30 am

Thanks for the update. CT
Charlie Tolman
Judson Supercharged
 
Posts: 1198
Joined: Wed Mar 01, 2006 10:36 am
Location: Minnesota

Re: Playing about with inlet manifold

Postby Asolepius » Fri Feb 19, 2016 1:59 pm

I've had this running for a while and can say that there is a noticeable improvement in torque. This is what I wanted all along, so worth all the effort. Next job is respraying the bonnet now the carb clears it.
Les Rose, Salisbury, UK
1959 Sprite, owned since 1978
Asolepius
1098cc
 
Posts: 489
Joined: Tue Jan 25, 2005 6:50 am
Location: Salisbury, UK


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