Home 
Home Search search Menu menu Not logged in - Login | Register
> Jensen Healey & Jensen GT Tech > Engine & Transmission > Puzzled by mist coming from Stromberg intakes

 Moderated by: Greg Fletcher
New Topic Reply Printer Friendly
Puzzled by mist coming from Stromberg intakes  Rating:  Rating
AuthorPost
 Posted: 07-27-2014 02:58 am
  PM Quote Reply
1st Post
srq2002
Member
 

Joined: 07-14-2014
Location:  
Posts: 23
Status: 
Offline
While under the bonnet adjusting the Strombergs I revved the engine and saw what appeared to be a light white mist coming OUT of the carb intakes. What could this be?

Some local friends suggested I check my pcv valve, but does a '74 JH have one? How does the crankcase vent?

Back To Top PM Quote Reply  

 Posted: 07-28-2014 04:38 am
  PM Quote Reply
2nd Post
answerman
Member
 

Joined: 09-10-2012
Location: Little Chute, Wisconsin USA
Posts: 382
Status: 
Offline
Couldn't tell you what the mist is, but... the factory setup to vent the crankcase was a hose from the rear of the engine that connected back to the factory airbox.

Now, if 40 years later yours is like almost every other JH I've seen, the factory airbox and plenum got ripped out and replaced with K&N pancake filters. In that case, the usual solution (since there's no way to connect that hose back to the K&Ns) was to stub the hose out and shove a little K&N filter in the end of the hose.

Last edited on 07-28-2014 04:39 am by answerman

Back To Top PM Quote Reply

 Posted: 07-29-2014 01:51 am
  PM Quote Reply
3rd Post
srq2002
Member
 

Joined: 07-14-2014
Location:  
Posts: 23
Status: 
Offline
Thanks, that's the info I needed on the breather. Think I may try to put together the parts to reinstall the factory intake setup.

Back To Top PM Quote Reply  

 Posted: 07-29-2014 07:41 am
  PM Quote Reply
4th Post
answerman
Member
 

Joined: 09-10-2012
Location: Little Chute, Wisconsin USA
Posts: 382
Status: 
Offline
That's what I ended up doing. Everything I read was that while there might be a slight performance bump with the K&Ns (or decline, there are a lot of opinions) with the European spec Dellortos, with the Strombergs it apparently doesn't make much difference at all. So, I restored the factory airbox setup because (a) I had all the parts, and (b) I have to say it is a great attention getter at car shows. All I have to do is park her with the bonnet open and within minutes I have people crowding around her asking why I have a muffler under the bonnet.

Back To Top PM Quote Reply

 Posted: 08-01-2014 07:08 pm
  PM Quote Reply
5th Post
Esprit2
Member
 

Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 293
Status: 
Offline
srq2002 wrote:
While under the bonnet adjusting the Strombergs I revved the engine and saw what appeared to be a light white mist coming OUT of the carb intakes. What could this be (Snip)...It seems this thread has gotten away from the original question.

There's a difference between crankcase oil fumes/ smoke, and fuel mist. I'm not sure which you're seeing. Hold a tissue in the mist, then sniff it. Does it smell more like petrol, or oil fumes/ smoke.

If fuel mist:
It's easy to think of inlet flow as being straight inward, but there's some very funky flow dynamics going on in the manifold. That includes reverse flow and reversion waves. The reversion waves can be strong enough to result in a standing wave of fuel vapor just outside of the inlet.

The strongest visual example I recall seeing was watching the old Can-Am cars go roaring buy. They'd touch 200 mph by the end of the front straight at BIR, yet there would be a standing cloud of mist about the size of a loaf of bread, stationary above the air horns. Well, stationary at 200 mph... the point is the cloud traveled with the car and never blew backwards. The cloud was due to reversion waves in the inlet runners.

Have you ever seen a picture of a fighter jet breaking the sound barrier, and there's a donut cloud of vapor surrounding the fuselage. The vapor cloud is due to a compression wave, it travels with the plane and doesn't get blown back by the supersonic wind. Loosely similar, but different.

Fuel injection can be more tolerant, but with carburetors, if the reversion wave blows back into the throat, it can play havoc with fuel metering. Manifold and carb designers use a number of tricks to try to control the reverse flow and reversion waves.

On the old Weber IDA 2-bbl down-drafts, as used on the Ford GT-40, there was an obvious smaller diameter tube sticking out of the middle of the air horn, and about 3/4" above it. That was an extension of the secondary venturi inlet that allowed it to breath smooth air from out beyond the reversion wave. That smoothed the airflow through the venturi and made jetting much easier.

Some Weber DCOE / Dellorto DHLA tuners apply the same trick, using K&S brass tubing to fabricate venturi inlet extensions.

Lotus' own die cast soft mount spacer has a tapered bore, like a funnel. On the down stream end, it's smaller than the carb or manifold bores, and looks like it would be a restriction. But going backwards, upstream, it forms a dam to the reverse flowing boundary layer, and interrupts it before it gets into the carb throat. It might be a restriction, but it's overall benefits outweigh it's losses.

Anyway, without seeing what you saw, I'm just suggesting it might have been something more esoteric than crankcase breather fumes.

Back To Top PM Quote Reply  

 Posted: 08-06-2014 06:34 pm
  PM Quote Reply
6th Post
srq2002
Member
 

Joined: 07-14-2014
Location:  
Posts: 23
Status: 
Offline
Evidently this is a phenomenon known as intake reversion, and is some of the cylinder contents flowing back the intake line (see, for example, http://www.circletrack.com/enginetech/ctrp_1008_intake_reversion/

It appears to be a timing issue, so I'll be checking mine. I'm curious if JHs generally do this. I think you need to rev the engine without the intake filters to see it.

Back To Top PM Quote Reply

Current time is 02:16 am  
> Jensen Healey & Jensen GT Tech > Engine & Transmission > Puzzled by mist coming from Stromberg intakes Top




UltraBB 1.172 Copyright © 2007-2011 Data 1 Systems