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Decceleration backfire but only RH side
Hi all,
This just started happening on my SPF/460, and not all the time. It all started when my RH sidepipe support bracket broke (common, I'm told) welded it, broke again, finally welded a bent rod "spine" under it. My collector bolts were lose so I attempted to tighten as best I could. It's really tight in there. No breaks so far, but the backfiring has started. Only as I lift off on the throttle as I shift to the next higher gear. Is this air getting into the exhaust? Do I have to redo the manifold gasket and collector gasket? Is there a quick fix? Thanks:( :( |
Yo Ray,
Nine times outta ten it's an exhaust leak. Extra oxygen gets into the exhaust flow somewhere after the exhaust port and mixes with the unburned fuel. Happens on deceleration. BOOM! Figure out where it is leaking and tighten or replace the gasket as required. I always try tightening first (don't overdo and break something off). Seems to be a common Cobra disease caused by vibration and heat :eek: Tom |
Or, just do like I do with my beater Porsche 912 whose exhaust system is just barely attached to the motor...
...Tell anybody who looks at you funny that the noise is due to the severe valve overlap of your full race cam dumping raw high test fuel into the hot exhaust. Sounds very impressive, and requires almost no work on your part. |
Ray - I agree with Tom. It's probably air getting into the exhaust, and from what you said, probably from a loose collector flange or a blown collector gasket. Because of heat and stress cycling, those bolts get brittle pretty fast and are easy to break. Some people have reported good luck with stainless steel bolts for this application. I had the same problem as you and replaced my bolts with grade 8 bolts with double nuts. So far, so good.
Hope this helps. |
Ray,
Just gone through the same problem with my EFI 302 motor. Slight loss of power associated with a misfire when changing gear (heard through one exhaust only). Replaced exhaust intake gaskets and plenum gasket and cleaned injectors - now running as it should. Problem with mine was that I don't have a diagnostics plug on the engine resulting in a fair bit of searching to find the cause. Air getting into the exhaust results in a lean engine which is over compensated by the fuel mixture thereby producing the misfire and excessive fuel usage (no longer cheap in Ozzie land) Good luck mate |
Hi CWizard,
Sorry but I need to disagree about the stainless hardware. Every time I've used those, they break off. And if they don't break, they seize so you can't get 'em off without twisting them off. There are regular steel bolts made for collectors' exhaust temps and acidity that are reusable, and re-tightenable (is that a word?). I use those. I prefer the Stage 8 header locks, too, while I'm recalling hardware specifics. I tried the Breslins, but they tried to ruin my aluminum heads' threads, and a couple of their allen screws which were supposed to lock them on, fell out! Either bolt style is hard to install unless you are plastic man, but the Stage 8s seem to hold fine for me. Tom |
backfire on deceleration
Ray..this also happened to me.
On the side of the backfire you need to check intake manifold gasket, header gasket to cylinder head, header flange gasket to exhaust pipe. One of these gaskets will all cause the backfire Mine was the intake gasket !! Tony |
Whew! Intake gasket would be easy enough although it could get expensive as it would be the best time to go for that twin 4bbl kit that is available from Carl's Ford Parts for $1695 :MECOOL:
I guess the thing to do is to re-tighten up first. I don't think it's been re-done after break in anyway. Anyone have torque specs on manifold bolts, intake and exhaust? Thanks for all the replies. I'm dreading the flange gasket replace ordeal.:( |
Hey Tom - Thanks for the comment about stainless steel. I've never used them but have been told by a couple of folks that they worked ok. Maybe they had low expectations! But the real gem in your post is the info on special steel bolts made for applications like our collectors. Please tell me more - what are they and where do I get some?
Many thanks! |
Hi CWizard,
Here are some: http://www.jegs.com/webapp/wcs/store...tegoryId=13923 or these: http://store.summitracing.com/partde...5&autoview=sku I've also heard of putting anti-sieze on the SS bolts but haven't tried it. I use the plain steel variety and they work OK! Tom |
Ray
With your cylinder heads aluminum I would say 20-25 ft/lbs on the intake bolts. Header flange is by hand so use your judgement. I ordered the SPF header flange to sidepipe gasket through D. Othloff, since SPF make there own exhaust I was not sure if there gaskets were universal !! Tony |
A new header to sidepipe gasket/collector gasket and ALL NEW Grade 8 STEEL nuts/bolt should fix you right up...
When your hanger broke, the pipe weight, along with the loose bolts to the headers/collector probably tweeked your gasket and retightening would no longer be a good "fix".... |
Tom - Thanks for the info. I'll have to try some of these next time I change out the collector bolts.
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Thanks Tony,
I'll call Dennis tomorrow and order them. I have to talk to him about installing the steel baraided brake lines I got from him. I can't get the old lines off the Wilwood calipers. I think they epoxied the originals in there. |
Ray
I would have to agree with PJS50, since your rear sidepipe bracket broke and the flange bolts were loose, change that gasket and you should be all set. I have one gasket if you need one in a hurry !!! Better that then the intake !!! Tony |
Thanks for the offer, Tony. I won't be able to get to it until next week anyway so I'll just order from D.O. I also ordered some new nuts and bolts from Jegs.
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Update...gasket replaced, still pops.
After a few weeks of procrastinating, Olthoff Racing finally sent me the gasket for the header flange to collector joint. The old gasket showed where the leaks were, around the boltholes going towards the center. It appeared that the center "cross" created by the header tube ends were slightly recessed, thus creating sort of a concave cavity in the center of the gasket.(The other side was flat) So my friend Joe had this tube of Wurth exhaust gasket material that hardens like cement. I put enough of it around the center to mold it into somewhat of a flat surface throughout. Bolted up, let dry for 48 hrs.
Today I ran a very short time trial at the Vanderbilt Cup in Roslyn and she still pops upon letting off the throttle and after hard acceleration. How can I test for leaks around the exhaust manifold area? I suspect the leak may be there. Any more ideas? BTW, here's how she looked dressed up for the event, stickers, numbers, handcut slicks on BRM wheels and all.:) http://www.clubcobra.com/photopost/d...edium/BRMs.jpg |
Ray ,are those BRM's off the CAV? I have an extra set for my CAV and was wondering if they would fit the Kirkham. chuck
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Quote:
They're new ones from Vintage Wheels and probably exact CAV duplicates. I got them to mount the racing tyres set for the CAV actually. Bob Lacey assured me they were an exact fit for the CAV. He was the original distributor for CAV GTs in the west coast. I wanted them also for the SPF double-duty, and the backspacing was 1/2" off (out), which suited me fine as the SPF wheels are set well in the wheelwell anyway. Upon trial fitting, I needed a pair of longer hub adapters for the rear, which Bob has in stock. When I go back to the road wheels, I have a pair of 1/2" spacers for the rear, or just mount them stock, just a bit more thread on the hub showing. http://www.clubcobra.com/photopost/d...dium/BRMs3.jpg |
Ray,
When you had your headers/sidepipes apart, did you look at the header flange to see if any of your individual cylinder pipe ends had cracked at the flange junction? I found a couple of cracks there that I had welded up when I did a rebuild... That can also be a source for air in your exhaust I assume your sidepipes were fine at this same location if you filled in that recess with that Wurth material...? Pat |
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