Carb Problem - Something's sticking

[BrandonBJ8]

After finishing my brakes, may car wouldn't start. It's been having a hard time starting for a while now. I removed the dashpots from the carbs and saw that the choke wasn't moving the jet at all. I jiggled it a few times and got it to where it was moving again. I put everything back together and started the car. It started right away, but it jumped up to 10k rpms and started slowly rising. I turned off and checked, but nothing seemed stuck. Tried again, same problem. Any ideas?

Thanks!
-Brandon

 

[BrandonBJ8]

Okay, I just noticed that one of needles is sticking through the rubber top of the cover. It was not like this before. Is this my problem? What could have caused it? How can I fix it?

Thanks!
-Brandon

 

[BrandonBJ8]

Here's a picture...

 

[BrandonBJ8]

And one more...

 

[100DashSix]

I'm new to this, but I'll give my opinion anyway. /ubbthreads/images/graemlins/grin.gif

Could the piston suddenly rising actually cause the damper to be forced through the top?? I'm going to guess you will have to buy a new damper. If those are HD carbs, MossMotors lists the part at $14.

If you remove the piston, does the jet move freely? If not, the jet will probably have to be removed and fixed. (Could it be a spring that has lost its tension?)

If when you turn on the engine the RPMs continue to rise and do not level off, I'd say it's a problem with the throttle linkage and/or butterfly valve(s). If the RPMs do stabilize (though a bit higher), I'd think it's caused by this sticking jet, enriching the mixture and causing the RPMs to rise slightly.

However I could be wrong on any or all(!) these points, so wait to see what a more experienced Healey member has to say.

 

[BrandonBJ8]

Well, I know one thing is correct, that I will have to buy a new damper from Moss; luckily only $14. I'm just not sure why this happened. When I had to put the cover back on, I had to give it more pressure than on the other one. Maybe something was just off and caused it to jam through the top. Any ideas?

Thanks

 

[kennypinkerton]

I'm wondering how that happened to... Did it backfire recently? What's the bluish looking stuff in the top of the damper cap? Was it like this before? Are you sure?

 

[Dave Russell]

This problem has certainly happened before. An intake spit back - backfire can raise the piston quickly enough to force the damper through the plastic top. In reality, the engine might run better with the plastic cap & damper temporarily removed for now. The remains of the damper are no doubt binding the piston & causing major problems.

The old brass tops were stronger & could better withstand this problem.

I agree with 100 DashSix on his suggestions. A new damper will likely solve the problem.

The covers should be reinstalled without the dampers. Lift the piston with your finger tip to look for smooth lift & free return to closed. Then with the oil chambers partially filled, & dampers carefully inserted, observing for any binding as the dampers are pushed down. Lift the piston with your finger tip to look for smooth lift with a little hydraulic resistance, & free return to closed.
D

 

[100DashSix]

Hm, when I screw in the dampers on my Healey there is usually a fair amount of resistance when the float on the end of it hits the oil in the dashpot. Steady pressure causes it to sink in properly, however. Could the additional pressure be because the dashpot had more oil in it?

 

[Keoke]

Yep Brandon, you are going to have to get a new damper.But lets fix that one temporarily until the new one cones.Remove the other damper and stand it on a level surface now adjust the length of the damaged one to exactly match the good in length.Turn the damaged one right side up and pour a bit of super glue in the hole and let it cure. Now remove the air cleaners and make certain that both pistons can be raised with a finger and make a distinctive clink when you release them and they hit the bridge.If one does not you may have a bent needle,dirty piston chamber or a misaligned jet bearing. Further, the choke activates a cam shoe.These cam shoes are affixed to brass rods that ride in a portion of the Carb body,it is not uncommon for these brass rods to stick in the carb body due to corrosion or excess dirt.Check everything out.---Fwiw---Keoke

 

[Keoke]

[ QUOTE ]
Hm, Could the additional pressure be because the dashpot had more oil in it?

[/ QUOTE ]

Normally this should not happen 100-6 as the tops should be vented.---Keoke

 

[Dave Russell]

[ QUOTE ]
Hm, when I screw in the dampers on my Healey there is usually a fair amount of resistance when the float on the end of it hits the oil in the dashpot. Steady pressure causes it to sink in properly, however. Could the additional pressure be because the dashpot had more oil in it?

[/ QUOTE ]
The end of the damper is actually a one way hydraulic valve. Resistance when the piston rises, free when it falls. Differences in initial feel could be caused by different oil levels. Once the dampers are fully inserted, any excess oil simply runs overboard. After the dampers have been inserted the first time, they should have the same feel. If not, something is not matched. Interchanging dampers sometimes helps.

Note: the pistons are originally a very precise fit in their housings & should not be interchanged between carbs. If something doesn't feel quite right, interchange the pistons to see if the fit improves. We could go into more detail about bent needles & sticky pistons if needed, but this covers the main operations.
D

 

[100DashSix]

Ah, didn't know that about the end of the damper, Dave. But, now that I think about it, it makes sense!

 

[tahoe healey]

The BJ8 should be vented in the black plastic cap. I cannot see the hole in your photo. Are these the originals? If you go to the brass be sure you vent them. The Moss plastic ones come undrilled but with a depression for the hole. I do not believe the brass ones are ment for the BJ8 but they look nice. I assume they are interchangable. I think that they many break over years of due to being improperly inserted after checking for damper oil. Careful pushing down and don't wiggle when re-inserting. They can crack easily.

 

[BrandonBJ8]

Dave - Are you saying that I the carbs can function without the dampers in?

Keoke - Thanks for the advice. I just tried adjusting the length of the damaged damper. I was able to push the rod back a little, but it's still slightly shorter than the non-damaged damper. Does it need to be exact? Can I still try it without being the same length? Also, how do you remove the air cleaners?

Also, could someone help me out on the terminology of everything. What exactly is the dashpot compared to the piston? I'm trying to fully understand all of the parts and how they work, but I'm getting confused there; especially when the Healey manual makes no reference of a dashpot.

Thanks,
-Brandon

 

[Keoke]

[ QUOTE ]
Dave - Are you saying that I the carbs can function without the dampers in?

Keoke - Thanks for the advice. I'm going to give that a shot. How can you adjust the length of the damper?

Thanks,
-Brandon

[/ QUOTE ]

You just pull it back in to the top until it is just as long as the good one and then glue it. NO !the carbs can not function correctly without the damper except as a short term trouble shooting test.---Keoke.

 

[Dave Russell]

[ QUOTE ]
Dave - Are you saying that I the carbs can function without the dampers in?
Thanks,
-Brandon

[/ QUOTE ]
Yes, the carbs could function without the dampers although not too well. It would be about the same as having no oil in them.

The SU carbs have no accelerator pump. When the throttle butterflies are suddenly opened, the dampers retard the piston's opening which serves to temporarily richen the mixture until the pistons reach their proper flow balance point. This takes the place of the accelerator pumps in other carburetors. The amount of throttle opening enrichment is controlled by the damper oil viscosity. Heavy oil is equivalent to more accelerator pump flow.

The dampers must be very mechanically straight & centered so that they present no mechanical drag to the piston movement. A temporary repair of a broken cap "could" work if the damper piston/shaft fits perfectly straight & concentric with the piston's oil chamber bore.

As others have said, With oil in the dampers, raising the piston in the carb intake with your finger should present moderate, smooth resistance when raising the piston & it should drop back down quickly with an audible click as it hits bottom. If the pistons move freely in both directions on this test with the dampers removed but not with dampers installed, & with no oil in them, the dampers are binding or bent.

Another source of piston binding can be that the lower jet is not centered. There is a procedure for jet centering.

To go a step further, a piston placed in it's inverted housing without the spring, should spin smoothly throughout it's range of up/down travel without contacting the sides of the housing. Sometimes tiny dents in the housing can cause contact. This can be corrected by carefully polishing off the high spots (dents) in the housing. As I said earlier, the piston & housing assemblies are carefully matched. If they get interchanged the fit can be too tight. Switching the parts can correct the lack of clearance.

To check actual piston to housing clearance for correct, there is a piston drop timing test that is listed in the manuals.

If everything was correct before the "misshap" a new damper/cap should correct the problem. It wouldn't hurt to check the rest of these items to assure that you have the best possible operation.
D

 

[piman]

Hello Brandon,

I have always taken the 'Dashpot' to mean the outer cover but it may correctly be refered to as the hydraulic damper which is what has broken in your picture.
I worked in electrical maintenace and have experience of current monitoring devices (overload protection) which had oil filled dashpots to give a delay setting before tripping the device.
This function is used in the S.U. (and Zenith) carburettor to prevent the piston lifting too quickly when you open the throttle, which would give a momentary weak mixture when you don't want it. So to answer another question, yes it will run without them, but you may get some spitting back when you accelerate.

Alec

 

[anthony7777]

brandonbj8, (www.hazelden.ca/austinhealey/index.html) ,will explain the functioning,and terminology associated with our beloved s.u. type carbs. /ubbthreads/images/graemlins/yesnod.gif

 

[zblu]

Havent twiddled with the chokes and got them off centre?
might be a cause of binding