Lotus Elan

Carb Air box bolts

PostPost by: StoatWithToast » Thu Jul 29, 2010 8:39 am

Hi,

After renewing all the carb plenum box bits a few weeks ago, I had to find a replacement bolt. Luckily I had one lying around that fitted.

Since then I've done a thousand miles or so, and have found that they keep coming lose. Unfortunately, even though I was tightening them up every chance I had going to and from Le Mans, the centre and back bolts both fell out and didn't end up in the useful valley of the wheel arch and pedal box make (one falling down here was the first idea I had that they were working themselves lose).

Anybody know what these bolts are?
Length, diameter, pitch?
Metric, UNF, UNC or some arcane bolt from the days of the empire?
Is this in the Lotus manual?

I'll use a little loc-tight once I have new ones now I'm back!

I think it's has been made worse by the carb to manifold parts all being old and either broken or on their way out. I'll be replacing these soon and hopefully that will stop some of the vibration that is undoing the bolts.

Cheers,

Dave
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PostPost by: pete d » Thu Jul 29, 2010 9:30 am

Hi SWT, carb air box bolts should be u.n.c quarter inch , try using spring washers ,always did the trick for me !
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PostPost by: StoatWithToast » Thu Jul 29, 2010 12:27 pm

Cheers Pete! :)

I'll give that a go, but don't the spring washers let air by?
Dave
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PostPost by: Galwaylotus » Thu Jul 29, 2010 1:15 pm

Any fluid, air included, will take the path of least resistance. With a nice big inlet, the amount of air possibly getting past spring washers isn't worth worrying about. I would guess you got a lot more through when the screws fell out! "The lesser of two evils"? IMHO
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PostPost by: StoatWithToast » Thu Jul 29, 2010 1:34 pm

Yeah, I know it as soon as they fall out!
Not only because they thumped against the under tray as the car flew over them, but the engine ran rough and hesitant the instant they fell out.
I will be using some tape to hold the box and and cover the holes to get it up to the garage I work in.
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PostPost by: billwill » Thu Jul 29, 2010 2:59 pm

All three are different lengths I think, to suit the fact that the air box tapers wider at the front.

Put a Fibre washer under the spring washer if you are worried about leakage there, or wipe a dab of silicone around them.

Seems that you need to balance the carbs if the engine is vibrating enough to shake them loose.
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PostPost by: Galwaylotus » Thu Jul 29, 2010 6:49 pm

StoatWithToast wrote:Yeah, I know it as soon as they fall out!
Not only because they thumped against the under tray as the car flew over them, but the engine ran rough and hesitant the instant they fell out.
I will be using some tape to hold the box and and cover the holes to get it up to the garage I work in.

I must be missing something. The carbs aspirate air through the airbox which is open at the front to allow air to be drawn in. How can the carbs "know" whether it comes in through the front opening ot through the airbox attaching screw holes? It shouldn't matter significantly whether the cover is on the air box or not, as long as the carbs are not starved of air the engine should run ok. What have I misinterpreted in the original posting? :?
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PostPost by: oldelanman » Thu Jul 29, 2010 8:08 pm

Galwaylotus wrote:I must be missing something. The carbs aspirate air through the airbox which is open at the front to allow air to be drawn in. How can the carbs "know" whether it comes in through the front opening ot through the airbox attaching screw holes? It shouldn't matter significantly whether the cover is on the air box or not, as long as the carbs are not starved of air the engine should run ok. What have I misinterpreted in the original posting? :?


If the air filter is clogged up air flow will be restricted and the engine will be running rich. Missing bolts and/or dislodged airbox will allow additional air to bypass the restriction and lean out the mixture. If this happens suddenly there may well be a noticeable change in engine performance.

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PostPost by: StoatWithToast » Fri Jul 30, 2010 9:43 am

I've got no idea why the engine hates the bolts not being in.
Maybe it disrupts the flow dynamics? Maybe the hotter engine area air messes with the mix?
All I know is that as soon as they fell out, I knew about it! I could drive around it to some extent with much slower throttle pedal progression, but even then it wasn't happy at certain RPM.

On the other hand, the car is over fueling, to the point where the sensor in the exhaust at the rolling road couldn't handle it, so the carb tuning is on the edge. Maybe it doesn't take much when it's set up like it currently is to make it rough.
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PostPost by: oldelanman » Fri Jul 30, 2010 10:19 am

StoatWithToast wrote:On the other hand, the car is over fueling, to the point where the sensor in the exhaust at the rolling road couldn't handle it


Have you checked your air filter ?

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PostPost by: StoatWithToast » Fri Jul 30, 2010 10:45 am

Yep - I recently put in a new filter housing, filter, ducting pipe and carb air-box cover.

Another thing the tuners found was that the timing was all over the place; they couldn't get a regular signal. I need to sort out the dizzy; replace the top end for a short term fix and get electronic ignition in there longer term.
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PostPost by: 512BB » Fri Jul 30, 2010 11:57 am

If your timing is all over the place, this is very often down to the timing chain either being to lose, or worn out. Try connecting up you timing light and look at the marks. If it is unsteady, stop the engine and tighten the chain 1/4 turn. Check the strobe again, and if it is then steady, or more steady, then the chain was to lose, and maybe tighten some more. If no change is observed, either your dissy has had it, or your chain is knacked, engine out! To do a good job it is anyway.

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PostPost by: StoatWithToast » Fri Jul 30, 2010 12:15 pm

512BB wrote:If your timing is all over the place, this is very often down to the timing chain either being to lose, or worn out. Try connecting up you timing light and look at the marks. If it is unsteady, stop the engine and tighten the chain 1/4 turn. Check the strobe again, and if it is then steady, or more steady, then the chain was to lose, and maybe tighten some more. If no change is observed, either your dissy has had it, or your chain is knacked, engine out! To do a good job it is anyway.

Leslie


Well, that's given me the fear! :lol:
Thanks for the advice. I'll have to get a timing light (we used the one at the garage last time).
Dave
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PostPost by: billwill » Fri Jul 30, 2010 12:24 pm

Sticky goo on the centrifugal weights in the distributor would also make weird spark timing.
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PostPost by: gjz30075 » Fri Jul 30, 2010 12:55 pm

In addition, worn bushing in the distributor can cause the same. Next time your have your distributor cap off, try moving the shaft back and forth. Shouldn't have any motion at all. A slight bit up and down is fine, though.

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