Honda-SOHC

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Why can't you have rusty petrol tank shot or bead blasted?

You can - nothing stopping you.  If you mean why isn't it a good idea that's different - and I can tell you that having had the outside of a tank sandblasted, I'd be very wary of trying to do the insides the same way. 

Long story, but I'd decided to get a tank powdercoated (yeah, long story . . . don't ask) and since the frame was going the same way, I just added the tank to the set of bits to get blasted.  Saw it halfway through the process and it looked absolutely fine; paint free and no rust either inside or outside.  Powdercoating completed and it looked great.  Things started to go sideways when I put fuel in it . . .

 . . . the grit was sharp enough to put two near microscopic holes in the underside.  Poking around confirmed there was no rot; the grit had simply found a couple of very slightly weak spots around weld seams and blown though.  Fuel and powdercoat really don't mix, and so I ended up needing to get the tank done again.  And then the rebuilt fuel tap leaked . . .

Suffice it to say that unless you're going to use a very soft media type or do something like vapour blasting, it's really not worth risking the cost of a paintjob simply to save a couple of hours work.  At the same time, the insides of tanks can incorporate some very odd shapes that aren't easy to blindly work around.  The tank I had done is from the mid-60's and has such a high middle section where it straddles the frame that it needs a connector hose to get fuel from one side to the other (hence the number of weld seams).  Working around that sort of thing would be a nightmare - and as others have pointed out, getting the media and the ultrafine dust it generates out when you're done would be more trouble than it's worth. 

You really don't want any of that getting into the engine . . .

Thanks for the explanation and evidence of your experience - i'm definitely not going to go down the blasting route!
Over the next few days when I have finished swishing my gravel about I plan to put some white vinegar I already have in stock to use before I proceed to electrolysis. I plan to use a multi phased approach as necessary to get a good finish.

Reading online reviews on Evaporust they talk about it leaving metal surfaces a dark /blackish colour.  I am very aware that surface rust soon returns so the idea of some sort of residual protective coating appeals. Its a shame that a reverse electrolysis cant easily be used at home to coat the tank surface with zinc or nickle.

As has been said put the air box & filter in place - it reduces the fire risk from a backfire.Even if you have the ignition timing 180 deg wrong the plugs will be wet  after cranking that will tell you fuel is getting through imho.
I assume you know there is some compression?
Likewise did you test for spark at the plugs?

Fuel, spark, compression ignition timing at the right cylinder - it's got to run in some sort of fashion.

Quote from: Macabethiele on September 04, 2020, 09:01:17 PM

Anyways I'm tickety boo today but am stuck until some bits & the Electrolyte arrives for my tank cleaning task. Still awaiting a sellers  response on a cheap Petcock.

What electrolyte are you waiting for? All you need is some bicarbonate of soda.

The other thing to try if you’re not in a hurry is white vinegar; it is very cheap (<£20 for 20L on EBay) and avoids all the hassle of fashioning electrodes shaped to reach the deeper recesses of the tank. The one advantage of electrolysis is that it only attacks the rust, not the good metal underneath.

Just pour the vinegar in and leave for a few days – depending on the degree of rusting – pour it out, but not down the drain. If you’re not happy just pour the vinegar back in and wait some more. If there is a lot of deposit in teh vinegar you can strain it using a kitchen sieve.

I bought some white vinegar a few weeks ago with a view to trying it - then read loads of reviews saying it does not work! We having it on our Fish & Chips instead. lol

Work has ground to a halt due to an overnight stay in local Hospital - I had Merlot posing as Urine then must have passed a kidney stone the pain was the worst I have ever had in my life - including a fractured femur when I was 18.
Had a scan of my kidneys yesterday waiting for the camera jobby to check my bladder out. Oh the indignity I have to face within 14 days as a day patient!

Anyways I'm tickety boo today but am stuck until some bits & the Electrolyte arrives for my tank cleaning task. Still awaiting a sellers  response on a cheap Petcock.

There's me thinking along the lines of OEM decided NOS must be the Japanese equivalent of "Blueprint" or "Quinton Hazel" then the penny dropped or should that be Yen clanged to the floor.

I managed to fit my Dynatek Coils using both old mounts but with some cutting as can be seen in the photo is the DS replacement coil longer ?

CB 400F2 coil mounts by Macabe Thiele, on Flickr

I guess it might be either an access issue or removing the waste but why not the above - I'm tiring of swishing my tank with gravel in it  every time I go in my garage.