Surging...or porpoising...or bucking. Pick your adjective.

[MarshallandMia]

Hi All,

Here's me chasing another problem again, and coming up empty handed.  I've been taking information from multiple car forums that all use SU carbs, and I've also had 4 mechanics to work on these bloody things.  

The issue is best described as self-perpetuating bucking.  The more I let it continue the worse it gets.  It seems more likely when cold.  Two ways to stop it: push the clutch, give it revs, and then go again, OR, throttle down and push through it.  It happens only at low rpms, often from a stop, going over bumps at low speed, or lightly pressing the throttle after coasting in gear.    

We have tested and retested all electrics and even swapped parts in and out (electronic ignition).  The carbs have been tuned and tweaked multiple times, and this last mechanic finally did the needles that I had been trying to get someone to do all along.  He apparently hones the needles himself?  

I don't know what else to do, except I came by this information on another forum while searching:

"check the jet height.  If the jet is too high, fuel will bounce in the jet and come over the rim of the jet and surge on every bounce.  The jet should be at least 0.060 below the bridge of the carb.  If you need the jet higher than that, get a leaner needle."

Does this mean anything to you all?  It seems to be a possibility based on the description, but at the same time I can't believe any of 4 mechanics wouldn't have caught this.

[yorkshire_spam]

So... if you take the tops of the SU carbs (dome, piston and needle) and look down from the top...

You should see the "bridge" the raised section in the bore of the carb, with a circular hole in the middle. Inside the hole, flush with the bridge you should see the top of the "jet bearing" (the bit the jet slides up and down in) and inside that you should see the top of the jet itself.

There should be a measurable height difference between the jet bearings (flush with the bridge) and the jet:

1 hour ago, MarshallandMia said:

The jet should be at least 0.060 below the bridge of the carb.

I've used the "tail" of a vernier caliper in the past to check it - it takes a bit of care AND you need to make sure the choke is fully home and the jets are fully raised (not by you touching them! By the action of the return mechanism) before measuring.

On this stolen image you can see the bridge and jet bearing, but the jet is not fitted:

[RRR]

I suspect your issue is more related to the original rod linkage. The bushes where the rod passes through the bulkhead can allow the rod to 'bounce' on light loading and provides an on/off action to the throttle spindles. This gets more pronounced if the throttle return springs attached to the manifold are soft.

Also there should be a return spring(28 +29) on the operating arm (14) which is there to alleviate this phenomenon. I've often found this to be missing and not always listed by suppliers but simple enough to replicate.

[MarshallandMia]

Hi guys,

Thanks for the responses.  To RRR, we have suspected perhaps that it had to do with the throttle assembly, without success, but I will dig further into where the linkage passes through the bulkhead.  There is a bit of play there.  I've replaced the manifold springs already, and the arm-to-chassis spring is intact and functioning properly.  

@Yorkshire_spam: When I'm feeling better (just had gallbladder surgery) I'll see what I can see regarding the jet level.  It seems as though I could probably see it, for a simply evaluation, through the intake, is that correct?

[mikeyb]

Hi Jim, as above, this is almost certainly down to the old rod accelerator linkages.  I had exactly the same problem on my Mk3.  The cure is to fit a throttle pedal and cable from a Mk4 Spitfire.  Easy fix with just one small hole to be drilled in the bulkhead (firewall)

I've not been over to the car shed yet since I saw you, but when I do I'll try and remember to take a few photos

Best regards

Mike

[sparky_spit]

As above..... someone borrowed my Mk3 Spit to do an autotest and completed the 1st course as if she was riding a kangaroo. On the second she just used full throttle or  brakes.

[MarshallandMia]

On 30/03/2022 at 11:42, sparky_spit said:

As above..... someone borrowed my Mk3 Spit to do an autotest and completed the 1st course as if she was riding a kangaroo. On the second she just used full throttle or  brakes.

This is absolutely hilarious! 🤣🤣 I can clearly picture it.  

Recent discovery on my trip to a Triumph specialist in Potter's Bar to have the carbs tuned correctly.  Apparently I've got some kind of a hot cam, and that is contributing to what he called 'hunting', and also the porpoising.  It is somewhat improved but he said it likely won't go away.  He did suggest upgrading air filters and maybe even carbs and needles to match the cam and the improved manifold/exhaust.  Surely not everyone with a Fast Road cam has this issue though??

I did check the play in the accelerator (there was some) and put a rubber bushing in there.  The play has now stopped, but it didn't seem to help much.  I'm not ruling it out, but right now I'm not going to go through the motions of fitting one from a Mk4.  I've spent WAY too much money for now...unless there's a free one out there???

[thescrapman]

Sounds like you need to drive with your foot hard tow the floor a lot more!